The Mosasauria

The cretaceous rocks of Europe and America have yielded another remarkable long-bodied marine Lacertilian, which attained a great size. This is the genus Mosasaurus, remains of which were first obtained from the Chalk near Maestricht.

Eighty-seven vertebrae belonging to one individual of this genus have been discovered, and when put together had a length of thirteen and a half feet. But there were certainly many more vertebrae than these, as those of the end of the tail are wanting, and there are gaps in the series of the rest. The centres of all these vertebrae are concave in front and convex behind; but the concavities and convexities are less marked in the posterior, than in the anterior, vertebrae. The atlas and axis are not well preserved in this series of vertebrae, but the nine following all have inferior spinous processes, which become shorter in the posterior vertebrae, and, in the last two, are represented only by a pair of low elevations. They have short transverse processes, each terminated by a simple costal facet. It is probable that these are cervical vertebrae. In the dorsal vertebrae, of which there must have been at fewest twenty-four, the transverse processes, which are strong in the anterior, gradually diminish in size in the posterior, vertebrae. There are no inferior processes. All the vertebrae which have been mentioned hitherto have the circumference of the centrum rounded, and are articulated to one another by zygapophyses. But a series of eleven, which follow them, have no zygapophyses, and the centra assume a more or less triangular prismatic form. The transverse processes of these are long, thin, and bent a little downward and backward. These seem to have been lumbar vertebrae. No sacrum has been discovered, but there are numerous caudal vertebrae with transverse processes, pentagonally prismatic centres, and chevron-bones attached to the middle of the under-surface of each. In the nine posterior of these caudal vertebrae the bodies are cylindrical, the transverse processes are obsolete, and the chevron-bones, anchylosed to the undersides of the centra, are long, inclined backward, and overlap one another. And, in the hindermost caudals, the spinous processes and the chevron-bones disappear.

There were strong ribs, but nothing is known with certainty of the sternum, limb-arches, or other bones.

The very complete specimens of the skull that have been discovered prove that its structure was very similar to that of the Old World Monitors in the large size of the nasal apertures, and the fusion of the nasals into a narrow bone. But sharp recurved teeth are anchylosed by their bases, not only to the premaxillary, maxillary, and dentary bones, but also to the pterygoid bones; and these pterygoid bones are unlike those of other Lacertilia, not only in form, but because they articulate together in the middle line for a considerable distance behind the posterior nasal aperture.