Part 4
Since the coracoids were developed outward and backward so much more than in birds, it would happen, from the apparent different direction of the second pectoral muscle, that the first pectoral muscle which in birds skirts the furculum, must have passed over the coracoid, probably pulling on its inside in opposition to the third pectoral. Either a subdivision of this muscle or a distinct muscle in the same place, in function corresponding to the subclavius muscle, appears to have been powerfully attached from the anterior prolongation of the keel of the sternum to the front face of the coracoid. It is improbable that the second pectoral muscle was undeveloped, but merely directed differently to what it is in birds, since, as will be seen, there is a process at the proximal end of the coracoid homologous with that which forms the pulley round which this muscle in birds works.
Professor Owen concludes his remarks by observing that the Pterosaurian breast-bone is in the main formed on the ornithic type. The muscles also appear to be similar to those of birds.
All the specimens are much mutilated, but all show the distinctive post-coracoid lateral emarginations, but as these are not seen in German Pterodactyles they are to be regarded as characters of a peculiar sub-order and not as characteristics of the sub-class.
The example figured in this memoir and by Professor Owen is 2-5/8 inches in antero-posterior measurement, probably about one third its entire length.
A small example in the collection of Mr Reed of York extends 1-1/4 inch in the same measurement, and by the analogy of _P. suevicus_ was more than twice that length when perfect. It is remarkable in that the coracoid facets look much less outward and much more backward than in the larger species.
The mammalian sternum is usually in many consecutive pieces like the vertebral column. The types in which it attains any size as an expanded shield are Cetaceans and the Manatee, but in these groups it has no keel and is not connected with the other bones of the pectoral girdle. The proximal portion of the sternum of the Mole is elongated and bird-like, with the shield narrower than in the typical gallinaceous birds, and with the keel similarly developed. It is connected with the humerus by small sub-quadrate bones named clavicles placed at the sides of the proximal end. The sternum in Bats usually consists of a proximal and a distal part. It is narrow except at the proximal-termination where it widens like the letter T or Y; and to the sides of the lateral prolongations are attached the long, slender, curved bones named clavicles, and a pair of ribs. This sternum develops a bird-like keel. Both Mole and Bat are regarded as differing from Pterodactyles in the bone giving attachment to the clavicles instead of to the coracoids. The proximal part of the sternum in both the living animals, gives attachment to but one pair of sternal ribs. The Pterodactyle sternum otherwise differs from the Bats in having the articulations for the coracoids close together, of a peculiar concavo-convex character, with a massive portion or keel prolonged forward in front of the coracoid articulations. The Bat cannot be said to resemble the Pterodactyle closely. The sternum of the Mole differs from that of the Pterodactyle in having a less developed shield, and in having a more developed keel which is not prolonged in front of the coracoid articulations. These examples demonstrate that resemblance in conformation is functional, and no proof of affinity.
Pterodactyles make some approach in the proportions of their sternum to Struthious birds. But the Struthionidæ have the bone thick, do not develop a keel, nor, have they an inter-coracoid process while the coracoid articulations are singularly long and narrow instead of being ovate. With other birds the Pterodactyle sternum agrees in giving attachment to the coracoid bones by synovial articulations, in the bone being shield-shaped, and supporting a more or less developed keel. The keel is chiefly developed at the proximal end, as in the Albatross, which has the bone broad; and it is prolonged in front of the coracoids exactly as in _Mergus merganser_, which sternum if a little broader in the shield and thicker in the keel would very nearly reproduce the sternum of the Pterodactyle, even to the "post-coracoid lateral emargination" of Cambridge specimens. Among reptiles the only form which suggests comparison is the Chameleon, in which however the sternum consists of an anterior and a posterior part as in the Bats, the back part narrow, and the front part a long lozenge shape, with a keel made by inclination of the sides of the bone to each other as in the Dodo, but the keel such as it is, is at the back part of the bone, and there is no prolongation in front of the coracoids as in Pterodactyle. The coracoids are broad, and are applied to the two anterior sides of the lozenge. The Crocodile has a narrow flat sternum which is prolonged anteriorly between the coracoids.
The resemblance is greater with mammals than with reptiles. From birds the Pterodactyle sternum makes no essential difference, and in the Merganser finds a close ally.
CORACOID.
Pl. 2, fig. 1-6.
Case. Comp. Tablet. Specimen. =J= _a_ 2 1-23
Commonly the coracoid in the Cambridge Pterodactyles is anchylosed to the scapula: occasionally the bones are separate, though the separation has hitherto only been observed in the largest species. In 1851 Professor Owen, when figuring the anchylosed ends of the scapula and coracoid in Pterodactylus giganteus (Bowerbank), observed that in no part of the skeleton does the Pterodactyle more nearly resemble a bird than in the scapular arch; a view again urged emphatically in 1859 when similar fragments were described from the Cambridge Greensand. Since then perfect examples of the coracoid have occurred, which show the characters given in the following description.
The bone is long, with sub-parallel sides, sub-triangnlar in section, with the proximal end expanded exteriorly and posteriorly, resembling in form the coracoid of a bird. The front surface looks forward and outward; it is flattened, is a little convex transversely, and a little convex in length; it is rugose with muscular attachments, which terminate in a tubercle on the uppermost fourth of the front, usually near to the inner side. The middle third of the slightly concave inside margin of the front aspect, is sharply angular; the parts above and below it have the angularity rounded off. The outside margin, a little more concave than the inside margin, is sharply angular in its distal third, in which the front gradually widens to near the sternal articulation, when it contracts--the whole sternal termination of the bone being directed a little inward towards the manubrium of the sternum. The inside, which faces the opposite coracoid, is convex transversely in the lower half or two-thirds; its distal termination is carried inward. The expanded proximal end of the inside is flattened, or channelled, by the developement inwardly, at the proximal end of the ridge formed with the front side, of a long strong process homologous with that on the inner side of the coracoid in birds. The channel so formed rounds on to the proximal surface of the bone, and extends backward to the limit of the scapula; over it the second pectoral muscle may be presumed to have worked[N]. The third side of the bone is much more concave in length than either of the others; it looks backward, outward, and downward, the proximal end being turned outward and downward more than the distal end; it is a little concave transversely at the expanded proximal end. Near the distal end there are sometimes visible a few faint marks of the insertion of muscular fibres, but they are much less distinct than those made by the coraco-brachialis muscle in the corresponding region of the coracoid in birds. Throughout its length it rounds into the inner side, and the upper third rounds convexly into the front. On the most posterior part of this aspect of the proximal end is a groove terminating in a long pneumatic foramen, partly in the coracoid, partly in the scapula.
[Footnote N: The homologous process is more developed in Pterodactylus giganteus. See f. 7. pl. XXXI. Owen, Cret. Rept.]
The muscular attachments on the front aspect of the coracoid appear to be two; one large and long inserted into the inner half of the middle third of the bone, terminating at the proximal end in a tubercle. No specimen shows the distal end of the insertion. This may indicate a subdivision of the first pectoral muscle. The other insertion, if it be distinct, is long and much narrower and at the distal end of the bone. This, according to the analogy of birds, should be the third pectoral muscle; if the insertion should be but part of that to which it is distally adjacent, then the third pectoral muscle must have had an enormous developement unparalleled in birds.
The distal end of the bone terminates in a synovial articulation concave transversely, convex from front to back, in form transversely ovate: the narrow side of the articulation, like the thin edge of the coracoid, being exterior. The articulation is about three fourths of the transverse diameter of the distal end; it is at right angles with the long axis of the bone, and looks downward and a little backward.
The proximal end, massively enlarged outward and backward, presents on the proximal surface three well defined regions. The largest of these is an irregular flattened surface half ovate in form, inclined to the axis of the bone at about 45°, looking backward, and upward also, when the bone is held vertically; the mesial hindermost half of the radius of this area is occupied by a pneumatic cavity: to this surface is applied the scapula. The next largest surface is rectangular and oblong, looking upward, outward, and a little forward. The transverse aspect which looks outward being nearly half as long again as the antero-posterior aspect which looks forward; in the latter direction the area is slightly concave, in the former direction it is slightly convex; its posterior boundary is parallel with the front of the bone: this area forms the anterior moiety of the glenoid cavity, to which the proximal end of the humerus is applied.
The remaining surface of the proximal end is sub-quadrate, adjoins the two other surfaces as well as the front and the inside of the shaft, it is conically concave.
The entire bone when applied to the sternum looked outward, backward and upward.
Professor Owen remarked (1859) that the "coracoid is shorter and straighter in birds than in Pterodactyles, but is commonly broader, and with a longer and stronger anterior process."
The points in which the Pterodactyle coracoid resembles that of birds (e. g. Gallinaceæ) are the long slender triangular shaft; the concavo-convex articulation to the sternum; the convexity of the distal end in front, and its concavity behind; the posterior aspect of its scapular surface, and the pneumatic foramen.
The points in which it is distinct from birds are that the bone is not produced proximally beyond the glenoid cavity for the humerus, which, instead of being lateral as in birds, and looking outward, in Pterodactyles forms the proximal-termination of the bone. The sternal articulation is proportionally much shorter transversely in Pterodactyles, terminating in a convex margin which rounds up into the thin outer margin, as in the immature coracoid of the common Cock. It is bow-shaped in front instead of being straight, and is commonly longer than in birds. The usual ossified connection with the scapula is not entirely unparalleled in birds, the whole pectoral girdle being sometimes anchylosed into a bony mass as in the frigate bird.
In the monotremata, the only mammals in which the coracoids are separate bones, they rather recall those of Ichthyosaurus than those of any other animals, and have no connection with the sternum. The bone which represents it functionally in placental mammals is the clavicle.
In no reptile is there any structure resembling the Ornithosaurian coracoid. The nearest approximation is made by the Crocodile, in which as in the Chameleon the pectoral girdle is formed as in pterodactyles and struthious birds by scapula, coracoid and sternum. But in the Crocodile the coracoid is compressed, and expanded from side to side both proximally and distally. Distally it has no synovial articulation with the sternum; and proximally a wide process of the bone extends beyond the articulation for the humerus as in birds, only the scapula unites with the prolonged part, and the glenoid cavity looks forward and inward.
The coracoid is essentially avian in its affinities, though with peculiar characters of its own. In the German genera it closely resembles specimens from the Cambridge Greensand.
23 specimens are exhibited. Nos. 4, 10, 12, are the middle parts of shafts of left coracoids. Nos. 3-12, 22, are the middle parts of shafts of right coracoids. Nos. 2, 5, 14, are proximal ends of left coracoids. Nos. 1, 6, 8, 9, 23, are proximal ends of right coracoids. Nos. 15, 16, 17, 18, 19, 20, 21, are distal ends of left coracoids. No. 13 is a nearly perfect left coracoid, and No. 7 is the glenoid cavity for the humerus formed by a right coracoid with the anchylosed scapula.
SCAPULA.
Pl. 1, figs. 2-12.
Case. Comp. Tablet. Specimen. =J= _a_ 3 1-17 4 1- 6 5 1- 4
Professor Owen described the scapula of Pterodactylus giganteus in 1851, and added further particulars regarding the Species from the Cambridge Greensand, in 1859; but, as with the coracoid, only the humeral end has hitherto been figured. The only example sufficiently perfect to give the length and proportions of the bone is preserved in the collection of Mr Reed, of York. This left scapula is a stout strong bone, short in proportion to its strength, of flattened ovate form in section, expanding at the humeral end into an irregular sub-rhomboid mass. It is smaller in the middle, contracting both from side to side and from back to front till the back to front measurement is 7/16 of an inch, and the side to side measurement is 11/16 of an inch, and it expands a little at the free end, which terminates in a smooth heart-shaped surface, convex in the long diameter, which measures 7/8 of an inch, and flat in the short one, which measures nearly 5/8 of an inch; it is at right angles with the inside of the bone. The sharp superior lateral outline is concave, but less so than the inferior lateral outline; into that inferior aspect of the bone the sides are more fully rounded. The flattened inner surface applied to the ribs is concave in the length of the bone, which measures 3-1/2 inches; the posterior half of which is convex transversely, the anterior humeral half is concave transversely so as to be cup-shaped, and measures in extreme width 1-11/16 inch; the outline of the transversely convex outer side in length is nearly straight, but the exterior part and glenoid cavity of the proximal end is broken away, and there only remains a small median proximal sur&ce broken at both ends, a little concave in length, measuring 5/8 of an inch, and convex in breadth measuring 1/4 of an inch.
As there is no specimen in the Woodwardian Museum showing clearly the connection of the proximal with the distal end, the specimens are arranged on separate tablets.
Humeral End of Scapula.
The humeral end of the scapula exhibits in the different species much diversity of form, spreading laterally from the shaft, and terminating in an elongated articular surface truncating the bone nearly at right angles. On its inferior border it throws out a large convex tuberosity, separated from the humeral articular surface by a deep emargination. From the tuberosity usually arises a crescentic row of muscular insertions, which is continued inward and forward over the most compressed part of the scapula towards the middle of the humeral articulation. From the superior margin, interior to the coracoid, arises a prominent ridge, the spine of the scapula, which is directed diagonally backward and downward, terminating in the middle of the outer surface, where it is bordered on the anterior aspect by a long narrow muscular attachment. Between this spine and the elevated margin of the glenoid cavity the bone is much compressed and concave.
On the inside surface of the bone there appear to be small muscular attachments in front of and behind the great tuberosity. The area between the spine and the inner surface is sometimes flattened, sometimes gently convex.
With well-marked distinctive characters in the inferior tuberosity, the pre-tuberous emargination and the thick rounded form of the bone, the Pterodactyle scapula is intermediate in character between that of a mole, a bird, and the crocodile; wanting the sabre shape of the bird's scapula, it also wants the wide expanded form of the scapula of the Crocodile, but resembles the latter in the direction and degree of developement of the spine. This modification is probably due to the outward direction and clavicular function of the coracoid, as well as to the raptorial habit of the organism.
In no living Reptile is there a scapula to be compared with that of the Pterodactyle, for besides the free end being expanded, in the crocodile, it is also thin and squamous and the bone makes a continuous curve with the coracoid as in struthious birds, and not a sharp angle as in Pterodactyles. The "spine" in crocodiles is on the anterior border of the bone and directed upward and backward, while in Pterodactyles it is on the posterior border and directed upward and forward. In the Chameleon the scapula is more elongated and narrow, narrower in proportion to its length than in Pterodactyle, but becomes rapidly wide at its union with the coracoid. It is curved in length so as to fit on to convex ribs. A scapula presenting some resemblance to Pterodactyle is found in certain Liassic Ichthyosaurs.
Among mammals a straight elongated narrow scapula is rare. The mole however has a scapula of this kind somewhat cylindrical in its proximal half and not much expanded at the free end, on which there is a small spine. The anterior emargination above the glenoid cavity in Pterodactyle is entirely mammalian, as is the anterior tuberosity above the emargination, for it entirely corresponds with what in ruminants, pachyderms and many mammals would be named the coracoid process. If that process is accurately determined it is difficult to say what this is.
In birds there is often a prolonged process on the inner side of the coracoid, which however extends interior to other parts of the scapula, and to this the furculum is attached. Such traces of a spine as are to be detected in the swan conform to the Pterodactyle.
No bird has the scapula cylindrical, even struthious birds only making an approximation to such a condition; and no birds have the scapula so straight. The bone is more avian and mammalian than reptilian; and more avian than mammalian but with strong distinctive characters of its own.
17 specimens of the humeral ends of scapulæ are exhibited. Nos. 1, 4, 6, 7, 8, 9, 11, 13, 14, 15, 17 are left scapulæ. Nos. 2, 3, 5, 10, 12, 16 are right scapulæ.
The tablet of the distal ends of scapulæ comprises 6 specimens.
Fore-Limb.
HUMERUS.
Pl. 4.
Case. Comp. Tablet. Specimen. =J= _a_ 6 1-46 7 1- 3 8 1
There are among the fossils of the Cambridge Greensand at least two well-marked _types_ of Pterodactyle humerus, readily recognised by the forms of the proximal and of the distal ends, and by the positions of the pneumatic foramina. In the group having the ulnar ridge developed the pneumatic foramen is on the posterior aspect of the bone[O] under the ulnar ridge, as in birds; but in some of the small Pterodactyles the foramen is on the anterior surface, and on its radial side. This latter kind of humerus has the distal end more or less divided into three convex surfaces, while the radial crest is enormously developed and terminates in a smooth oblong flattened surface nearly as large as the proximal articular surface, and looking anteriorly. The distal articular surfaces are not as in birds parallel to that of the proximal end, though they agree with those of birds in being at right angles to the radial crest; this ridge in Pterodactyles being directed much further outward and backward than in birds.
[Footnote O: Professor Owen states (p. 16, 3d Supt.) that the foramen is palmar. Fig. 15. T. III. 2d Supt. shows it to be anconal.]
The largest forms of Pterodactyle all have the distal articular surface flatter, and the proximal articulation less bent back so as to look more upwards. No specimen of this kind of humerus has occurred with the radial crest preserved; but it is apparently carried farther down the shaft and not so far forward as in the other group. This latter kind of bone is shown by Prof. Owen in T. III. figs. 1, 2, 3rd Sup. Cret. Reptiles; the former kind has been illustrated in figure 5 of the same plate.
Some of the most gigantic Pterodactyles appear to have had the limb-bones as solid as those of crocodiles, and unpermeated by air; and there is no evidence that the high Avian characteristics of most of these Greensand fossils also pertained to all the previously known types from the lower secondary rocks.
The osteological series comprises 46 specimens. No. 30 is a nearly perfect right humerus. Nos. 1, 2, 5, 6, 7, 8, 9, 11, 18, 22, 23, 25, 39 are examples of the proximal ends of left humeri. Nos. 3, 4, 10, 12, 13, 14, 15, 16, 17, 19, 24, 26, 27, 28, 38, 40, 41 are examples of the proximal ends of right humeri. Nos. 20, 21, 32, 33, 34, 35, 37, 42, 44, 45, are examples of the distal ends of left humeri. Nos. 29, 31, 36, 43 and 46 are distal ends of right humeri.
No. 30 shows the entire length of the humerus to be 2-1/2 inches. It has a nearly circular shaft with a diameter of a little more than a quarter of an inch, being more slender than the corresponding bone of Pt. suevicus, which has the same length. The _proximal_ articular surface is crescentic, the anterior concavity corresponding with the concave anterior aspect of the proximal end, while the convex border corresponds to the convex posterior side of the bone, which it overhangs: it is worn, but appears to measure half an inch from the radial to the ulnar side. The ulnar ridge (which is worn) has not extended more than a quarter of an inch beyond the articular surface. The thin bird-like radial crest, arising rather more distally than the ulnar ridge, is flat on its posterior surface, and extends anteriorly for a distance nearly half as far again as the length of the proximal articular surface of the humerus. On the proximal third of the posterior face are two contiguous long narrow oblique muscular insertions. The proximal ends Nos. 22, 23, 24, 25 are examples of this kind of bone, having the pneumatic foramen radially situated on the anterior aspect near to the articular surface, as may be seen in No. 24. No. 25 shows the termination of the radial crest in an oblique oblong smooth surface, slightly convex in length and breadth, directed distally towards the ulnar side.
No. 6, 7, 13, 27, are examples of another kind of proximal end, where the pneumatic foramen is an oval hole on the ulnar side of the posterior sur&ce. The radial crest arises more distally, and the ulnar ridge more proximally, than in the small species, like No. 30.