CROCODILIA. 365 



From the shortness of the humerus, and the indications that 

 the vertebral column was unusually flexible, and that the tail 

 was laterally compressed, it was early conjectured that the 

 Mosasauroids were marine and aquatic in their habits. This 

 conjecture has been raised to the rank of a certainty by the 

 discovery that the fore and hind limbs of the Mosasauroids 

 were in the form of fin-like paddles, like those of the Ichthyo- 

 saur and Plesiosaur. There can therefore be no doubt that 

 Mosasaurus like the living Amblyrhynchus was aquatic in its 

 habits, and frequented the sea-shore, coming, in fact, only oc- 

 casionally to the land. The best-known genus is Mosasaurus, 

 of which the most celebrated species is the M. Camperi (fig. 

 316) of the Maestricht Chalk. Other genera belonging to this 

 group are Leiodon, Baptosaurus, and Halisaurus. Recently, 

 Marsh has described bony dermal scutes as present in several 

 Mosasauroids (e.g., Holcodus, Leiodon, and Edestosaurus], thus 

 rendering their Lacertilian affinities doubtful. 



In the Tertiary Rocks the remains of Lacertilians are not by 

 any means unknown, but none of the forms of this period are 

 sufficiently important to demand especial attention. Most of 

 the Tertiary Lacertilians, however, are of small size, and ap- 

 pear to have been terrestrial in their habits, thus approximating 

 to the typical existing Lizards. 



ORDER IV. CROCODILIA. The last and highest order of 

 the living Reptilia is that of the Crocodilia, including the living 

 Crocodiles, Alligators, and Gavials, and characterised by the 

 following peculiarities : 



The body is covered with an outer epidermic exoskeleton 

 composed of horny scales, and an inner dermal exoskeleton 

 consisting of squared bony plates or scutes, which may be con- 

 fined to the dorsal surface alone, or may exist on the ventral 

 surface as well, and which are disposed on the back of the 

 neck into groups of different form and number in different 

 species. The bones of the skull and face are firmly united to- 

 gether, and the two halves or rami of the lower jaw are united 

 in front by a suture. There is a single row of teeth, which are 

 implanted in distinct sockets, and hollowed at the base for the 

 germs of the new teeth, by which they are successively pushed 

 out and replaced during the life of the animal. The centra of 

 the dorsal vertebrae in all living Crocodilia are procoelous, or 

 concave in front, but in the extinct forms they may be either 

 amphicoelous (concave at both ends) or opisthocoelous (con- 

 cave behind). The vertebral ends of the anterior trunk-ribs 

 are bifurcate. There are two sacral vertebrae. The cervical 

 vertebrae have small ribs (hence the difficulty experienced by 



