Part 7

Each os innominatum shows a hemispherical acetabulum which is slightly elongated in antero-posterior extent In the Dimorphodon the bone which is superior to the cup, that is to say, which extends dorsally along the sacral vertebræ is prolonged anteriorly as a strong narrow straight style, the base of which is seen in the parts marked _Ilium_ in Nos. 1 and 6. A more perfect example may be studied in a pelvis from the Cambridge Greensand preserved in the collection of the Geological survey. Posterior to the acetabulum a similar but stronger bony style extends for more than the length of the acetabulum, curving slightly downward at its posterior part. The dorsal outline of this portion of the bone is slightly concave. The posterior horn like the anterior horn forms part of the ilium which constitutes the upper half of the acetabular cup. The os innominatum contracts in antero-posterior extent below the acetabulum, and immediately widens again in a thin concave bony expansion. The anterior or pubic outline is comparatively straight, and at right angles with the ilium; the posterior or ischiac outline is deeply cupped where the ischium unites with the ilium, and becoming straight extends backward at a considerable angle. The ischium contributes less to the pelvic cup than either the ilium or pubis; it is flat in front and convex on the visceral side, rounding into the narrow flattened posterior side. The pubis is separated from the ischium by a suture extending vertically through the obturator foramen. The obturator foramen [seen in No. 9] is small and oval, less than half the diameter of the acetabulum, situated below its ventral border. It passes obliquely downward and a little forward, and its opening makes the exterior aspect of the pubis concave; the visceral aspect of the pubis is convex from side to side like the ischium. The sacral aspect of so much of the os innominatum as is seen, is concave from the dorsal to the ventral margins, and is cupped behind and below the acetabulum, the surface being rough. Among reptiles the ilium is chiefly behind the acetabulum, in mammals it is chiefly in front. In the over-lapping group, Aves, it extends both ways. Among the Amphibia the ilium is chiefly anterior to the acetabulum. In Crocodiles it has a slight extension both ways, in Dinosaurs the extensions are more marked and the whole arrangement approximates to birds. But among animals which have been affiliated with reptiles the Dicynodonts are the only order in which there is a pelvis so mammalian and massive. If the ilium of the Monotreme genus Echidna had a posterior extension, the pelvis would be altogether comparable with the pelvis of this Pterodactyle, and would differ chiefly in the larger obturator foramen, the perforated acetabulum and the unanchylosed condition of the pelvic elements. The pelvis of Apteryx does not make any near approximation.

Moreover specimens Nos. 3 and 4 show on the anterior pubic border, about the base of the acetabulum, a slight pit or roughness to which something has been attached, and in the original specimen of Dimorphodon associated with the pelvis are two triangular bones which recall something of the form of the prepubic bones of Echidna. Most German Pterodactyles show on the OS pubis an enormous prepubic bone. In Iguana the pubis forms at its anterior border, a sharp angular process. In Chelydra the process is long and narrower, and arises from the middle of the border. In Echidna this prepubic process has become a distinct prepubic bone and is more elongated. Unlike the marsupial bones it is attached to the pubis by a wide base. The anterior pubic roughness of Cambridge specimens, and the loose bones of the Dimorphodon, &c. indicate the existence of structures in the Ornithosauria homologous with the prepubic bones of the Ornithodelphia.

So far as it is comparable with living animals, the ilium is altogether avian, differing in being narrower; and the pubis and ischium are mammalian.

The upper anterior corner is the most elevated part of the acetabular border, as in the great Auk and some birds of vertical position of body, and many mammals.

FEMUR.

Pl. 8.

Case. Comp. Tablet. Specimen. =J= _b_ 11 1-16

Twenty-six specimens are mounted to illustrate the Femur. 10 are proximal ends; 16 distal ends. But in the series illustrative of species is an entire specimen of a right Femur 4 inches long. Fragments Nos. 3 and 12 show proximal and distal ends twice as large, but most of the examples are about the size of the entire femur.

It is a straight sub-cylindrical bone, flattened in front, a little compressed from front to back distally, and (in one type) compressed proximally from side to side behind. The distal articulation has a broad shallow channel passing down from the front and imperfectly separating two condyloid parts, which extend a little backward and are divided behind. The outer condyle extends a little outward, and so gives the outer side of the bone at the distal end an oblique compressed aspect like that which prevails among birds and many mammals. Proximally the shaft contracts suddenly and is produced upward, forward, and inward as a rounded neck, as long as in the femur of any mammalian carnivore, which expands rapidly at the end to form the hemispherical ball, which articulates with the pelvic acetabulum.

No. 1 shows a well-marked pit for the ligamentum teres at the back part of the ball. At the proximal end of the shaft below the neck is a large pit for the obturator muscle, and at the outer front angle a great trochanter. Proximally the bone can only be compared with the mammalian Carnivora, Quadrumana and Man; distally it is avian and mammalian.

In one genus exemplified by specimens 5-10 the obturator pit is not developed.

Sometimes the shaft is curved a little convexly, outward and forward.

TIBIA.

Pl. 8.

Case. Comp. Tablet. Specimen. =J= _b_ 12 1-11

Eleven specimens are mounted to illustrate the tibia, of which 1 to 9 are regarded as proximal ends; and 10, 11 with less confidence are regarded as distal ends from which the distal epiphysis has come away. It is to be anticipated that the distal end of the tibia in Pterodactyle will when found approximate to the distal end of the tibia in the bird.

The bone is at the proximal end straight and sub-cylindrical, slowly enlarging proximally; convex behind, except for an elevated boss some little way below the proximal articulation for the attachment of powerful muscles. In front the shaft is a little flattened proximally, with a mesial groove dividing two prominences which are apparently homologous with the ridges below the patella in birds. The proximal articular surface truncates the shaft at right angles except at what is regarded as the outer front aspect, where it rises into a small patelloid prominence.

It shows the impressions of two condyles, which correspond in form with the distal end of the Femur.

Nos. 3 and 6 are regarded as left tibia; Nos. 4, 5, 7, 8, 9 as examples of right tibia.

No specimen likely to be a fibula has been found. In Dimorphodon and in German Genera the fibula is Avian in form. The Crocodile offers some approximation to the Pterodactyle shape in the proximal end of the Tibia, but the Pterodactyle has Avian characters in addition. Its straightness and length, ridges on the front and patelloid prominence, are Avian.

TARSUS OR TARSO-METATARSUS.

Pl. 8.

Case. Comp. Tablet. Specimen. =J= _b_ 13 1

This fragment, which may be the distal end of the bone corresponding to that called in birds the tarsus or tarso-metatarsus, is badly preserved. Yet so close is its resemblance in form, structure, and apposition of the constituent bones to what obtains among birds, that it may probably be identified as the tarsus; while the peculiar characteristics of Pterodactyle bones which it shows, demonstrate that it is not from a bird, but from an Ornithosaurian skeleton.

The bones are of paper thinness, and consist of a strong bone behind which distally appears on the inner side to be compressed and thrown backward and flattened at the side, exactly like the inner toe in Natatorial birds. On the front of this strong support, confluent with it, and confluent together, so that the places of union are only seen at the distal end and in transverse section, are three bones, together as wide as the bone on which they rest. It does not appear possible that the distal articulations could have supported more than three digits.

This bone, if correctly determined, offers points of affinity with birds as pronounced and as important as any thing shown by the extremities, for among reptiles a welding of the (tarsal or) tarso-metatarsal bones is unknown, and here it is as absolute as in any bird, and takes a characteristic bird shape. In the Rodent Jerboa the metatarsus has much the same form as in a bird.

No phalanges have been recognised.

The Vertebral Column.

ATLAS AND AXIS.

Pl. 9.

Case. Comp. Tablet. Specimen. =J= _c_ 1 1-15

Fifteen specimens are mounted to exemplify the structures of the Pterodactyle atlas and axis. Nos. 1, 11, and 2 have already been figured, and described by Prof Owen, the latter as a section of a cervical vertebra.

The _atlas_ centrum, a saucer-shaped disk of bone, commonly united more or less intimately with the centrum of the axis, but sometimes free. It presents in front a hemispherical cup for the basi-occipital, and is flattened or slightly convex behind. Its neural arch is seen in Nos. 2, 10, and 12; but the only specimen with the arch entire is in the museum of James Carter, Esq. The neurapophyses vary in form and size, but always are small obliquely flattened lamellar bones, which extend upward and backward to meet the neural arch of the axis, just above the neural canal, where a thin and small cross piece connects them together.

The distinctive aspect of these bones is given by the neural arch of the _axis_, which is very much elevated, and is formed by two flattened sides, which meet in a vertical ridge above the neural canal, and look forward, outward and upward; extending laterally more and more beyond the side of the centrum, but not reaching so far back as the posterior articulation of the centrum. Each side of the neural arch at its middle part behind is produced into a thick obliquely flattened process, the under portion of which shows the small posterior zygapophysial facets, which look downward, outward and backward. The lateral outline of the part of the neural arch above this process is concave; as is the lateral outline between it and the centrum. Behind, the neural arch is concave, and looks a little backward. The neural canal is stirrup-shaped in front, but is higher and sub-ovate behind. The neural arch of this, as of all the other vertebræ, except a few dorsals, is inseparably united to the centrum, without a trace of the line of union. In the middle of the side of the vertebra, and at what may be presumed to be the union between the neural arch and the centrum, in a concavity, is the pneumatic foramen. It is round or oval, and varies in form and size though not in position. In No. 8 it exhibits the subdivided reticular structure characteristic of the pneumatic foramina of birds. In No. 10, which has a short centrum, the pneumatic cavities are reduced to a few small perforations, no larger than would be made with fine needles.

The centrum is shorter than in cervical vertebræ, commonly convex (No. 8) on the visceral surface; often with a slight longitudinal hypapophysial ridge (Nos. 1; 7; 12) rarely flattened (No. 10). Towards the hinder part the centrum widens, and becomes concave on the visceral surface, sending off as do the other cervicals, below the transversely elongated posterior articulation, a pair of short strong apophyses.

The posterior articulation can only by a modification of the idea be said to conform to the cup-and-ball plan, for though convex from above downward and convex from side to side, the elongated transverse measurement would be three times the depth. On the under side an impressed transverse line divides the articulation from the concave part of the centrum below.

Cervical Vertebræ.

Pl. 9.

Case. Comp. Tablet. Specimen. =J= _c_ 2 1-43

Forty-three specimens are mounted to exemplify the variations in size and characters of cervical vertebræ. These for the most part are specific characters; and between the axis and the first dorsal vertebra the variations in an individual were slight. [Those nearest to the back, as in birds, are widest in front, and have the highest neural arches.] The associated series show commonly four cervical vertebræ behind the axis, and in two cases apparently five; never more. So that as seven appears to be the number of true cervical vertebræ in most if not all of the German Pterodactyles, it may be presumed that the Cretaceous Ornithosaurians also had this character in common, with Mammals, and probably as persistent. In Iguana there are 6, in Monitor 7, and in Crocodile 8.

The centrum is united to the neural arch as in birds, without a trace of suture; sometimes the neural arch is no wider than the centrum, sometimes it extends over the centrum on each side. Those forms with a narrow neural arch have the neural spine high, and its sides look forward as well as outward. The pneumatic foramen is oblique. An example is figured by Prof. Owen, in the memoir on Pterodactylus simus, pl. 2, fig. 4. The forms with a wide neural arch have the neural spine rising from the middle of the dorsal surface, erect and equally compressed from side to side. The pneumatic foramen is horizontal. An example is figured in Prof. Owen's memoir on Pt. simus, pl. 2, fig. 1. These two forms of cervical vertebræ may be regarded as typifying two genera.

In both forms many characters occur in common, and as the specimens illustrative of special modifications will be described hereafter, the following description has been made to embrace the chief characteristics of these vertebræ in Cretaceous Ornithosaurians.

The inferior aspect of the centrum is oblong (being narrower than long), or quadrate; when quadrate the additional lateral expansions are external to the pneumatic foramina, and are formed by the neural arch and zygapophyses. The centrum proper is a little wider in front than behind, and the side outlines are concave. The base of the centrum is flattened, or more or less hollow, or more or less tumid and regularly convex; in front there is often a mesial ridge, which never reaches the posterior articulation, and forms a prominent tubercle at the base of the anterior articulation. At the posterior end the outline of the centrum is concave, and mesially the bone has a hollow corresponding to the tubercle in front of the adjacent vertebra; and the part of the centrum on each side is prolonged slightly into a strong rounded or flattened tubercle below the side borders of the posterior articulation; these posterior processes, in vertebræ in situ, fitted, on each side of the mesial anterior process of the vertebra behind, on to concavities more or less marked. Analagous processes are developed in the cervical vertebræ of many birds.

All the Cretaceous Pterodactyles have the articular surfaces of the centrum transversely oblong, as have some birds. The posterior articulation is convex from side to side, and convex from above downward, and appears to extend a little further on this neural than on the hæmal surface; in outline it is commonly an elongated oval, but sometimes attends on the upper surface of the inferior lateral tubercles. The anterior articulation is transversely elongated, concave in both directions, and sub-triangular in outline; that is to say, the superior outline is more or less convex, and from its limits to the mesial tubercle at the base, the inferior outlines are more or less concave.

The neural canal is sub-circular or ovate in outline, and quite as large as the neural canal in vertebræ of Dinornis of similar size.

The neural arch like the centrum has commonly a depressed appearance. It always has a neural spinous process which is directed upward. In the depressed type the neural surface of the vertebra is in outline usually sub-quadrate, but concave at each side, and concave in front and behind; the four corners are the processes which support the zygapophysial facets, the surface is divided into two lateral parts by the strong neural spine. These lateral parts are from front to back flat, or slightly concave, or slightly convex; and from the neural spine outward they are always concave. The neural spine is commonly sharp in front and flattened behind. The neural arch is placed well forward, so that while a third of the neural canal remains uncovered by it behind, rarely a sixth would be uncovered in front.

The anterior and posterior zygapophyses are commonly connected by a more or less rounded ridge, undefined above, but well defined below, since under its posterior part at about the middle of the side of the centrum is placed the pneumatic foramen.

The anterior zygapophysial processes are separated from the anterior articular surface of the centrum by a more or less oblique channel. Towards the base of this channel in many vertebræ may be seen a small and short flattened antero-posterior perforation corresponding in position with the usually large perforation for the vertebral artery. If the passages are to be regarded as having subserved such a function, it will not be without interest to remark the small relative size of the cerebellum in these animals; since the vertebral artery conveys the blood to that region of the brain.

The anterior zygapophyses are strong processes directed forward and outward, compressed a little from side to side; they are placed at the outer sides of the anterior articular face of the centrum, and extend in front of it.

The zygapophysial facet is commonly oval and looks upward and inward and forward.

The posterior zygapophyses are short and massive, but otherwise correspond closely with the anterior zygapophyses, only with all the parts reversed, and except that necessarily they are relatively to the neural canal a little higher.

A sharp and well defined angular ridge, commencing at the back of the zygapophysis, is directed inward, and forward, and upward along the posterior margin of the neural arch to the top of the neural spine. The posterior aspect of the neural arch is concave from side to side, and makes a right angle with the superior lateral aspect.

The part of the centrum exposed behind the neural arch is convex above from side to side.

The pneumatic foramen between the centrum and the neural arch varies greatly in size; it is oval and longitudinal.

The largest specimens have the centrum 2-1/2 inches long; in the smallest the centrum measures 5/8ths of an inch in length.

In the second type of cervical vertebra the side of the centrum makes a right angle with the base, and is separated from it by a sharp angle as in struthious birds. The side of the centrum is concave, with a few small pneumatic perforations; and the side of the centrum, which is high posteriorly, rounds over the oblique ridge connecting the zygapophyses, into the oblique lateral face of the neural arch. The anterior zygapophyses are very large and the posterior zygapophyses small and placed as high as the top of the neural canal.

Every region of the vertebral column displays pneumatic foramina, situated as in the vertebræ of birds.

The large proportional size of the neck-vertebræ is common to some birds, and is conspicuous in many mammals, like the Llama. In most mammals where the vertebræ have a cup-and-ball articulation, the ball is in front, as it is in the dorsal vertebræ of the penguin, so that those vertebræ are not comparable with Pterodactyles, although on the under side of the centrum they similarly give off a mesial process below the cup, and a lateral process below the ball on each side. The neural spine in Pterodactyle is commonly more developed than is the case with long-necked birds or mammals. Reptiles such as Crocodiles and Lizards have the neural spines of the neck-vertebræ well developed. Birds differ from Pterodactyles in the peculiar articulation of their vertebræ. In both the centrum is often depressed, in both it is concave from side to side in front, and convex from side to side behind, but in birds it is also convex from above downward in front, and concave from above downward behind, while the reverse arrangement obtains in Pterodactyles. A similar condition to that of the bird is seen in the neck-vertebræ of the Kangaroo, of Man, and several mammals, only the vertical curves are less marked. Vertebræ concave in front, and convex behind, and devoid of cervical ribs, are met with among the Lizards, but neither Monitor nor Iguana offer any parallel to the form of the cervical vertebræ of Pterodactyle, which is best matched among Marsupials and Birds. Birds commonly have more vertebræ in the neck than have Pterodactyles, which in that respect resemble mammals and some Lizards.

Dorsal Vertebræ.

Pl. 10.

Case. Comp. Tablet. Specimen. =J= _c_ 3 1-20

Twenty specimens are mounted to exemplify pectoral and dorsal vertebræ. Like the cervical vertebræ, they include two types of form, one with the centrum flat, figured in pl. 2. fig. 20-22 of the memoir on Pterodactylus Sedgwicki, and regarded by Prof Owen as anterior dorsal; and the other form with a convex centrum, figured 24-25 of the same plate of the same memoir, regarded by Prof. Owen as posterior dorsal. Following the analogy of birds such determination is as well supported as the similar reference of the two types of cervical vertebræ to anterior and posterior parts of the neck, but fuller materials compel a reference of the two types of dorsal vertebræ to two different genera.

Nos. 1, 3, 6, 7, 8, 9, 10, 14, 15, 19 belong to the flat type. Nos. 2, 4, 5, 11, 12, 16, 17, 18 exemplify the convex type.

Dorsal vertebræ are rare fossils; and in the associated sets of bones never more than four dorsal vertebræ are found, rarely more than one. No specimen of the type with a convex centrum occurs in the associated sets.

The dorsal vertebræ with convex centra have all lost their neural arches except No. 2. The form of the centrum is half a cylinder, as long, or longer than wide, but sometimes depressed, and wider behind than in front. The exterior surface is smooth, convex from side to side, and slightly concave from front to back. The neural surface is mesially excavated. Both anterior and posterior articular surfaces are semicircular or sub-ovate, being wide from side to side.

The anterior articulation is cupped, concave from the neural to the hæmal surface, and concave from side to side. The posterior articulation is convex from the neural to the hæmal surface, in which direction, it usually shows striations, and from side to side has a gentle convexity, sometimes so slight as to be nearly flat.

The neural canal is large, ovate, and as high as is the centrum.