443, MANUAD OF ZOtTLOGY. 



is no less than five feet long ; and as the tail and limbs were 

 formed for swimming, there can be little doubt but that Mosor' 

 iaurus — like the living AmUyrhynchus — was aquatic in its 

 habits, and. frequented the searshore. Several other Reptiles, 

 either belonging to the genus MosasaurusyOt nearly allied to 

 it, have been described from the Cretaceous and Tertiary 

 rbcks of North America. Recent researches oh these Mosa- 

 sauroid Reptiles by Professor Marsh of New Haven have 

 shown that they possessed fin-like paddles like those of the 

 Ichthyosaur and Plesiosaur. 



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 epiderrtiic 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 vertebras in all living CrocodUia 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 vertebrte. The cervical 

 vertebrte have small ribs (hence the difficulty experienced by 

 the animal in turning quickly) ; and there are generally false 

 abdominal ribs produced by the ossification of the tendinous 

 intersections of the recti muscles. There are no clavicles. 

 The heart consists of four completely distinct and separate 

 cavities, two auricles, and two ventricles, the ventricular sep- 

 tum — as in no other Reptiles — being complete. The right and 

 left aortae, however — or, in other words, the pulmonary artery 

 and systemic aorta — are connected together close to their origin 

 by a small aperture {foramen Panizzce), so that the two sides 

 of the heart communicate with one another. The aperture of 

 the cloaca is longitudinal, and not transverse, as in the Lizards.- 

 All the four limbs are present, the anterior ones being penta- 

 dactylous, the posterior tetradactylous. All are oviparous. 



