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.
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.