Part 8
The neural arch is strong, compressed from back to front, and placed as usual on the anterior part of the centrum. In outline it is sub-rhomboid with the sides concave. There is a strong process on each side above the neural canal for a rib, and apparently a neural spine, but all are broken. The transverse processes for the ribs are directed outward, and a little forward, flattened in front and behind, the surfaces being sub-parallel, so that in front the neural arch is concave from side to side. Behind, the neural spine is directed between the transverse processes so as to over-hang the exposed part of the superior surface of the centrum. At the points where the neural spine bends back from the transverse processes are the posterior zygapophyses, high above the neural canal, and parted from each other by an interspace as wide as the canal is high. They look downward, outward, and backward. The lateral surface below the transverse process is narrow, flattened, bends at a right angle with the posterior surface, rounds into the anterior surface, is a continuous curve with the side of the centrum, and is concave from below upward. The superior surfaces of the neural arch have the sides sub-parallel, they are each concave from side to side; and these surfaces are excavated for pneumatic foramina.
Dorsal vertebræ of the type with the centrum flattened closely resemble cervical vertebræ with the centrum flattened, differing chiefly in the less length of the centrum. Occasionally (as in No. 3) the neural arch comes away from the body of the vertebra.
The centrum is very depressed, sub-quadrate, and wider than long; the base is flat, or slightly concave, with occasionally a slight longitudinal mesial ridge; the lateral outlines are concave, so that the bone is pinched in from side to side. The neural surface of the centrum is flat and parallel with the base, and, as usual, wider behind than in front, but the centrum is not there so high. The surfaces for the neural arch are flat, and extend nearly to the base of the centrum in front, so that they look upward, outward and a little forward.
The articular ends are remarkable for their depressed oblong character, still preserving the anterior concavity with a small mesial process below, as in cervical vertebræ, and similar but smaller processes at the inferior outer angles of the posterior sub-semicylindrical convexity. The middle third of the anterior cup is made by the trapezoidal anterior end of the centrum; sometimes the sutures between it and the neural arch are well marked.
The neural arch is large, commonly with a sub-circular neural canal. The neural spine is high, compressed so as to have the lateral surfaces sub-parallel and rounding into each other superiorly; and it has a less antero-posterior extent than the centrum. At its base behind it widens rapidly, and forms massive quadrate processes, extending outward and backward, which on the outside each have a flattened ovate zygapophysial facet, which also looks downward. Above the facet and separated from it by a groove is a tubercle. Between the zygapophyses behind the bone is concave from side to side; the facets are placed above the neural canal.
The posterior zygapophyses are placed considerably higher than the anterior zygapophyses, and the part of the neural arch between is rather constricted from front to back. The neural arch steadily widens in front down to the base of the anterior zygapophysial processes in such way that the more or less flattened lateral surface looks outward and is gently concave from above downward. A ridge commencing at the tubercle over the posterior zygapophysial facet descends in a curve forward and downward, to form the posterior border of the anterior zygapophysial process. This is separated by a groove from the anterior articular surface, and anterior part of the base of the centrum, and has the aspect of a compressed part of the neural arch, extending obliquely downward, and forward, over and beyond the articular surface of the centrum. The anterior zygapophysial facets are oblong, narrow from side to side, and long from front to back; they are directed forward and a little outward, and are flattened, make nearly a right angle behind with the front of the neural arch, and look upward and inward. They are sometimes placed as high as the top of the neural canal, but are commonly lower. Around the neural canal the bone is conically impressed.
Minute pneumatic foramina are in the usual position, between the centrum and the neural arch; and sometimes others behind the anterior zygapophysial process.
The largest specimen known has the centrum nearly an inch and a half long.
The dorsal vertebræ in Cambridge specimens would appear to make a nearer approximation in number to birds than to Mammals or Lizards or Crocodiles, though Chelonians have few vertebræ in the back. Among Reptiles the form of the vertebra makes some approach to that of the Monitor, Chameleon and Scink. In most Mammals the dorsal vertebræ have the centrum convex, though in the lumbar region its visceral surface often becomes flattened. But though very unlike there is a nearer resemblance to the lower dorsal vertebræ of a Struthious bird.
Sacrum.
Pl. 10.
Case. Comp. Tablet. Specimen. =J= _c_ 4 1-7
Seven specimens are mounted to exemplify the ordinary structures of the Ornithosaurian sacrum.
Nos. 1 and 2 have the centrum convex, exactly as in the dorsal vertebræ of the convex type. Nos. 3-7 have the centrum flattened, following in general features the plan of the dorsal vertebræ with flat centra.
No. 1 is a vertebra from a sacrum, where perfect anchylosis had not been induced; it has the neural arch well preserved, and shows the sharp suture which united it to the preceding vertebra.
No. 2 shows two entire vertebræ and part of a third, which have lost the neural arches but have the centra perfectly anchylosed together. The middle vertebra measures 5/8 of an inch in length, and at the suture from side to side measures more. The surface is smooth, regularly convex from side to side, and gently concave from back to front. The last vertebra shows the articular vertebral surface; it is convex in both directions, and oblique, so that a large part looks upward. The anterior of the three vertebræ is pinched in at the lower part of the sides of the centrum. No. 1 shows that the neural surface of the centrum is deeply excavated, making the neural canal an elongated upright oval. Above the centrum, which forms only the middle third of the articular surface, the neural arch expands on each side into a wedge-shaped transverse process, the lower surfaces are flattened, and continuous with the centrum, while the upper surfaces are flat and horizontal as in birds and Dinosaurs, and form the platform from which arises the massive neural spine.
In front the transverse wedge is flattened and compressed, so as to look forward and outward, and in the middle shows a large ovate pneumatic foramen. Behind, the wedge is compressed so as to look backward and downward.
The neural spine is massive and forms rather more than half the height of the vertebra. It is flattened with a ridge rising near its base in front and ascending in a concave curve obliquely backward and upward. The anterior parts approximate a little in front, while the small parts posterior to the ridge approximate a little behind. The sides of the neural spine approximate superiorly, and appear to round into each other.
There is a notch on each side in front at the base of the neural spine, and another above the central articulation. The neural spines appear to have been united by suture. It may be instructive to compare the neural spine just described with the specimens =J=. _c_. 10.
Of the second type or genus No. 4 to 7 all show the anterior cup for the last lumbar vertebra. No. 3, 5 and 6 all show two entire vertebræ and part of a third preserved, but no specimen shows the posterior termination of the sacrum. No. 7 has the articular face of the centrum very broad, and greatly depressed. In No. 6 it is ovate and has the neural arch preserved; above a semicircular neural canal it sends out on each side a short horn-like zygapophysial process. No. 4 is remarkable for the small size of the circular neural canal, the centrum when entire measuring an inch from side to side, while the neural canal only measures 5/16 of an inch. No. 5 is figured by Prof. Owen. No. 4-6 appear to have given off transverse processes from the sides of the centra. No. 7 appears to widen into transverse processes at the point of suture between the centra.
In No. 3 the base of the sacrum is flattened, and its sides pinched in, and concave in outline from back to front. In this hollow are small pneumatic foramina, and between the hollows the vertebræ widen in the line of the suture so as to send out strong short transverse processes or tubercles. Above the hollows are given out the strong horizontal quadrate pyramidal transverse processes. All their sides are flattened or a little concave, and the under side displays a large ovate pneumatic foramen. Each of the four angles of the transverse process gives off a ridge. The lower ones descend obliquely to the anterior and posterior intersutural tubercles. The upper two ascend obliquely, in front and behind, and form rounded ridges on the neural spine. The neural spine is flattened, moderately compressed from side to side, and cupped a little over each transverse process. In front the neural spine is flattened transversely and perpendicular; the transverse processes are also flattened and a little in advance of the neural spine.
The sacrum in its general aspect is Mammalian. In the Bird the vertebræ are much more numerous and do not retain their individuality so well. In Reptiles properly so called, the sacrum never includes more than two or three vertebræ, and those commonly remain unanchylosed. But in almost any placental Mammal in which several vertebræ are anchylosed together, a sacrum similar to that of the Pterodactyle is met with. No mammalian sacrum, however, is furnished with pneumatic foramina.
Caudal Vertebræ.
Pl. 10.
Case. Comp. Tablet. Specimen. =J= _c_ 5 1-13
Thirteen specimens are mounted to exemplify the osteology of caudal vertebræ. No. 7 has been figured by Prof. Owen in the memoir on Pterodactylus simus, pl. 2 fig. 13-16. The centrum of the largest specimens measures one inch and a quarter long, and the vertebra is half an inch wide from side to side in the middle. The smallest specimen No. 13 has the centrum 3/4 of an inch long. The vertebræ vary in proportions, some being much more slender than others. They present a close approximation in form to the first type of cervical vertebræ, differing chiefly in being more elongated.
They are elongated bones constricted in the middle, so that the outlines of the sides seen from above or below are gently concave; the outline of the anterior end is sub-rhomboid, the outline of the posterior end is sub-pentagonal, as would be a transverse section of the vertebra. The long outlines of the base of the centrum and of the top of the neural arch are sub-parallel.
The two sides of the upper surface of the neural arch are smooth, flattened, a little concave from back to front; they are inclined to each other pent-house wise at about a right angle, and are separated throughout their length by a narrow slightly elevated neural spine. Behind, the neural arch is truncated transversely so as to expose the posterior neural surface of the centrum, which is convex from side to side. The outermost lateral angles of the neural arch are the posterior zygapophysial processes, short and strong above the centrum, with a tubercle on the upper surface, and showing the sub-circular zygapophysial facets behind; they look backward and downward, and are separated by a groove from the region of the centrum. Under the sharp ridge which connects these zygapophyses behind, the neural arch is excavated, and the cup shows the termination of three canals. The largest one is the upright oval of the neural canal in the middle, on each of its sides separated by a narrow bony wall is another perforation, very variable in size and shape, sometimes b& large as the neural canal, but usually small and circular. The anterior end of the neural arch is cut into, so that as seen from above, the straight sharp anterior margins diverge mesially from each other at a right angle, and so expose to view a small anterior part of the neural surface of the centrum. These lines are prolonged forward and outward, to form the upper margin of the anterior zygapophyses, which are compressed and prolonged over and beyond the sides of the anterior articulation, from which they are separated by a slight groove; the anterior and posterior zygapophyses are connected by a rounded ridge. The anterior end of the neural arch is excavated, but less so than the posterior end; in the middle is the oval perforation of the neural canal, and at the sides other perforations corresponding to those behind are placed a little in advance of the neural canal. The anterior and posterior articular surfaces differ in no respect from those of cervical vertebræ.
The inferior surface of the centrum is separated from the sides by two ridges parallel to the lateral concave outlines of the neural arch; they extend from sides of the front, more or less well marked, to the tubercular processes at the base of the sides of the centrum behind. The dice-box shaped area of the centrum so inscribed is usually concave from front to back, and concave from side to side behind, and convex from side to side in the middle; this convexity is only broken in front by the development of the slight mesial hypapophysial ridge.
The sides are narrow, flattened, look downward and outward, are a little concave from front to back, round into the centrum and into the neural arch, and show at about the middle a small pneumatic foramen, which is variable in size, but largest in No. 8, and sometimes a mere puncture.
The caudal vertebræ differ in many ways from other animals. They have neither transverse processes, neural spines, hypapophyses or hæmapophyses. In the persistence of the neural arch down the tail they resemble reptiles and birds rather than mammals, in which nothing but the centrum persists to the end of the tail. The vertebræ are furnished with vertebral arteries which run through the neural arch parallel to the neural canal, in exactly the same position as do the vertebral arteries in the neck-vertebræ of the Llama.
THE BONES OF THE HEAD.
Pl. 11, 12.
The skull of Dimorphodon differs in form and in many important details of structure from that of Rhamphorhynchus; and both of these types of skull are strikingly unlike that of the short-tailed animals named Pterodactyle. Hence, as it will be shown that the Cretaceous fossils of this class belong to very distinct new genera, there is no reason for assigning to them by anticipation any class of cranial structures. The cranium of this type of animal has never been critically described, and for all that is yet known to the contrary Pterodactyles may differ between themselves as much as birds or mammals. Their affinities have been unknown. Therefore, before describing bones it may be desirable to state the grounds on which the several specimens are referred to the Ornithosauria. The fossils on which this section of the memoir is founded are, the basi-occipital and basi-temporal bones, the anterior portion of a cranium, the back parts of four crania, facial bones, and the quadrate and quadrato-jugal.
The crania are all no larger than that of the Heron; though from the Greensand are bones and jaws indicating Pterodactyles both smaller and larger. The skulls are mostly remarkable for wanting both basi-occipital and basi-temporal bones. And the specimen of basi-temporal and basi-occipital corresponds posteriorly with the Pterodactyle atlas, anteriorly with these crania; it is hence concluded to have belonged to a similar animal. Being relatively twice as large, it indicates that in these animals the basi-occipital condyle was proportionally larger than in known birds; and that animals of a cognate kind had skulls probably twice the size of these. The anterior basal part of the hinder sphenoid terminates in a remarkable triangular surface, with two perforations, which are separated by a median ridge. Almost entirely corresponding with this is the basal surface of the anterior part of a cranium, fractured in front of the pituitary fossa. Therefore, and as it indicates a similar capacity of brain, it is regarded as belonging to the same kind of animal as the others ; but being five times the size, it must, if the proportions of the Heron were preserved, have been part of a head a yard long.
Now, as there is no other animal with the same texture of bone, or exhibiting with high organization the same diversity of size, these cranial fragments are referred to the jaws and bones of Pterodactyle. So marked are their structures that many quarry-men refer vertebrate fossils to their several orders with almost as much accuracy as would a practised anatomist.
Basi-occipital and Basi-temporal.
Pl. 11.
Basi-occipital, Owen, _Sup. Cret. Rep._ p. 6, T. 1, figs. 11, 12, 13.
Case. Comp. Tablet. =J= _c_ 7
This bone was not found associated with any set of fossils that would induce us to refer it to one species more than to another. Its Ornithosaurian character was probable; and Prof. Owen described it in his last memoir on the Greensand Pterodactyles.
But though indubitably basi-occipital, it is so anomalous in some respects that the Professor regarded the under as the upper surface; since then the investing phosphate of lime has been removed, and the bone is now described in what appears to be its natural position.
Viewed from above the fossil divides into two parts; the occipital condyle, and an anterior, wide, transversely oblong extension terminating at each side in a strong short horn. The posterior half of the condyle shows large cancelli as though so much of it had been covered by the articular cartilage. The sides of the condyle converge, so that posteriorly it is only two-thirds of the width it has at the foramen magnum, which would appear to indicate a comparatively slight lateral motion of the head. The condyle is hemispherical posteriorly and superiorly; there is a depression between it and the great foramen of the skull; inferiorly it is flat.
It is 7/16 of an inch long; posteriorly 9/16 wide, nearly 6/16 of an inch high anteriorly. It terminates in front superiorly in an elevated transverse ridge.
On removing the matrix, the anterior surface of this occipital bone was found to be concave; yet as nothing but cancellous structure is seen it may be but imperfectly ossified or more probably, imperfectly preserved. And the bottom of this cup expands forward in a thin sheet of bone a quarter of an inch long and half an inch wide, which on the under side is continuous with the base of the condyle.
On each side of this floor and partly extending in front of it, and below it, is an irregular piece of bone, half an inch long, resembling anterior zygapophyses of cervical vertebræ.
Though in most vertebrates the basi-occipital enters into the basal floor of the skull, the median bones are either so placed that they rest one upon another from before backwards or abut against one another nearly perpendicular, so that the basi-sphenoid comes commonly to underlap and partly hide the basi-occipital. Nowhere among Amphibia or Reptilia do I know of the reverse position occurring. In some fishes there is an approach to it. Thus a slight anterior bony expansion of the basi-occipital in the Cod fits partly into a horizontal slit in the basi-sphenoid[A]. In the Carp the basi-occipital has a spathulate basal expansion like that of Pterodactyle, but it is underlapped by the basi-sphenoid[S]. In some mammals the under side of the basi-occipital extends further forward than does the neural side, as for example in the Sheep and Goat; while in a few others, as in the Walrus, the reverse positions obtain.
[Footnote S: Parasphenoid of Prof. Huxley.]
But it is among Birds that the structure described in Pterodactyle is evident and characteristic. For although the bony plate under the sphenoid,--Mr Parker's basi-temporals,--is mostly anchylosed to the bones about it, and less with the occipital than with others, its position and relations are quite the same as those of the expanded flap of this Pterodactyle basi-occipital. Therefore it is identified with the basi-temporal bones.
Back of the Cranium.
Pl. 11.
Case. Comp. Tablet. Specimen. =J= _c_ 8 1
This fossil is an inch high, rather wider, and half an inch long. It well shows the bones at the back of the skull, the basi-cranial bones, and the bones posterior to the frontals, which roof in the Cranium. There are in it striking resemblances to the back of the skull of some Natatores, as the Grannet and Cormorant, and of some Grallatores as the Heron, and Gallinaceous birds as the Cock.
_The base of the skull._ The bones here indicated are the basi-occipital, basi-temporal, and basi-sphenoid. The former two have come away as from an articular joint, and are wanting. The basi-occipital does not enter into the floor of the cranial cavity, and only rims the foramen magnum. But its basi-temporal expansion rests beneath the posterior part of the basi-sphenoid forming the base of the skull; its long convex anterior end fits into the concave groove at the back of the anterior part of the sphenoid. The squamous basi-temporal bone appears in this species to have been as long as the foramen magnum is wide, and to have been relatively thicker than in the other form already described.
The _basi-sphenoid_ is a thin expanded bone forming the floor for the cerebellum, and terminating anteriorly in a triangular mass, while the slightly convex part behind, covered with the basi-temporals, is nearly square. It enters into the foramen magnum, forming its lower part; and is confluent with the ex-occipitals behind, with the periotic, alisphenoid and perhaps with the squamosal at the side; and as in birds all these sutures are obliterated. This is probably the only instance in the Animal Kingdom in which the basi-sphenoid takes so important and singular a share in the functions of the basi-occipital bone. The anterior part of the basi-sphenoid projects below the posterior part, is nearly flat on the basal surface, and forms an equilateral triangle with the apex in front and base behind. In the middle of the triangular bone is a slight longitudinal ridge, and behind the middle of each outer side a rather large foramen which appears to be the inferior opening for the carotid artery. The triangular part is hollow and as long as the quadrate portion. The lateral parts of this anterior bone are nearly flat. They converge upwards and are rounded in front to form the boundary of the pituitary fossa, and do not appear to have terminated in a spine. Above are the alisphenoids.
_The upper part of the skull_ is divided into two segments by a strong straight transverse ridge, which leaves the occipital bones behind, and the parietal &c. in front.