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.
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.