REPTILES. 647 



oibit ; parietal foramen none ; alisphenoids large ; orbitoaphenoid radimentary or 

 wanting; vertebrse completely ossified, procoeloua except atlas, dentatus, the two 

 sacrals, and first caudal ; in extinct species amphi- or opistbocoelons ; cervical vertebrae 

 with small ribs ; ribs articulating with the vertebrae by means of a head and tubercle ; 

 always oviparous. 



Inhabit fresh water in hot countries. 



The Crocodilians are all extra-limital. The existing forms may be divided as 

 fellows : 



Muzzle large and flat. a. 



Muzzle elongated, rounded and dilated at the end. . . . Gavialid^. 

 a. Fourth tooth in lower jaw received into a notch on the side of the upper 

 maxillary ; hind legs with a toothed fringe, and toes completely palmated. 



Crocodilid^. 

 a. Fourth or canine tooth in lower jaw received into a fossa in the upper ; hind 

 legs simply rounded and toes semipalmate. . . . Alligatorid^. 



GavialidsB, the Gavials, comprise two genera, Cravialis, one species, G. gangeticus, in- 

 habiting the Ganges, and Somistoma, two species, in the rivers of Borneo and 

 North Australia. 



CrocodilidsB, the Crocodiles, has one genus, Crocodilus, with four American, three 

 African, four Asiatic, and one Australian species. On this continent they are not found 

 north of Yucatan, Guatemala, or Cuba, except one species, Crocodilus amerioanus, which 

 occurs in Florida. 



Alligatoridse, the Alligators or Caimans, comprise also but a single genus. Alligator, 

 with ten species, and are limited to the New World. 



Alligator mississippiensis or lucius is the common Alligator of our Southern States. It 

 is dark ash-brown above, paler beneath ; dorsal plates with elevations forming discon- 

 nected longitudinal ridges ; four of these oarinate plates upon the neck are arranged quad- 

 rately. 



