THK CROCODILIA. 221 



cartilage is interposed between the distal end of the astragalo- 

 navicular and the second metatarsal, and unites with the head 

 of the first metatarsal. 



As in the manus, the three, pre-axial, clawed, digits are 

 stronger than the others. The fifth is represented only by an 

 imperfect metatarsal. The numbers of the phalanges are 

 2, 3, 4, 4, counting from the tibial to the fibular side. 



In the Grocodilia the teeth are confined to the premaxillfe, 

 maxillae, 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 predeces- 

 sor. The teeth vary much in shape, having either long, curved, 

 and acute, or short and obtuse, or almost globular and straight, 

 crowns. Very often they possess sharp anterior and posterior 

 edges, which may be finely serrated. 



The Grocodilia 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 differ 

 but little from some of those which now exist, in the Mesozoio 

 sind Cainozoic formations. 



They may be divided into the following groups : 



A. With procoelous presacral vertebras, and posterior nares bounded 

 below by the pterygoids. (All existing 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 aperture. 



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-maxillary suture straight or convex 

 forward. The mandibular symphysis not extending beyond 

 the fifth tooth, and the splenial element not entering into it. 

 The cervical scutes distinct from the tergal. 



1. AlUgatoridce. 



Alligator. Caiman, Jacare. 



b. The head longer. The teeth unequal. The first mandibular 



tooth biting mto a fossa ; the fourth, into a groove, at the 

 Bide of the upper jaw. The premaxillo-maxiUary suture 

 straight or convex backward. The mandibular symphysis 

 not extending beyond the eighth tooth, and not involvmg 

 the splenial elements. The cervical scutes sometimes dis- 

 tinct from the tergal, sometimes united with them. 



2. Crocodilida. 



Crocodilus. Mecistops. 



