258 THE ANATOMY OP VEETEBEATED ANIMALS. 



In the Crocodilia tlie teeth are confined to the premaxillse, 

 maxillse, and dentary part of the mandible. They are 

 simple in structure, have large pulp cavities, are lodged in 

 distinct alveoli, and are replaced by others developed upon 

 their inner sides. The development of the new tooth causes 

 absorption of the inner wall of the base of the old one, and 

 the replacing tooth thus comes to lie within the pulp cavity 

 of its predecessor. The teeth vary much in shape, having 

 either long, curved, and acute, or short and obtuse, or almost 

 globular and straight, ci'owns. Yery often they possess 

 sharp anterior and posterior edges, which may be finely 

 serrated. 



The Crocodilia are to be found in the rivers of all con- 

 tinents and the larger islands in the hotter parts of the 

 world. None of the existing species are truly marine. , 

 though many of the extinct species were. They are first 

 known to occur in strata of Triassic age, and abound, 

 under forms which difi'er but little from some of those 

 which now exist, in the Mesozoic and Cainozoic formations. 



They may be divided into the following gi-oups : — 



A. With procoelous presacral vertebrae, and pnsteriornares bounded 

 below by the pterygoids. (All existinfr Crocodilia, and the fossil 

 forms of cretaceous and later formations, are included in this 

 division.) 



a. The nasals enter into the formation of the nasal aperlure. 



a. The head short and broad. The teeth very unequal ; the 



first and fourth of the mandibles biting into pits of the 

 upper jaw. The premaxillo-ma.xillary suture straight or 

 convex forwards. The mandibular symphj'sis not ex- 

 tending beyond the fifth tooth, and the splenial ele- 

 ment not entering into it. The cervical scutes distinct 

 from the tergal. 

 1. A.lligatorid(B. 



Alligator. Caiman. Jacare. 



b. The head longer. The teeth unequal. The first mandi- 



bular tooth biting into a fossa ; the fourth, into a groove, 

 at the side of the upper jaw. The premaxillo-maxillary 

 suture straight or convex backwards. The mandibular 

 symphysis not e.Ktendin-i beyond the eighth tooth, and 

 not involving the splenial elements. The cervical scutes 

 sometimes distinct from the tergal, sometimes united 

 with them. 

 2. CrocodilidcE. 



Crocodilus. Mecistops. 



