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.
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.