162 REPORT OF NATIONAL MUSEUM, 1898. 



II. No preorbital Joraraeo. 



a Dorsal hypapophyses rudiraental. 



Muzzle elongate, nasal bones not reaching nares. 



Holops Cope. 

 aa Dorsal hypapophyses elongate. 



/? Mandibular teeth projecting on each side of the maxilla; muzzle 

 elongate, nasal bones not reaching nares nor premaxil- 



laries Gavialis Cuvier. 



yS/5 Mandibular teeth received within the margin of the maxilla. 

 y Muzzle elongate, nasal bones not reaching nares. 



Nasal bones reaching premaxillaries Tomisloma Miiller. 



yy Muzzle short, nasal bones reaching nares. 



d Fourth mandibular tooth filling a notch in the upper jaw. 



No bony nasal .septum Crocodilus Laurenti. 



No bony nasal septum OsieolcBmus Cope. 



SS Fourth mandibular tooth received within the maxillary border 

 (one exception). 

 Claws three; a bony nasal septum; ventral scutes not or 



little ossified ' A U'ujator Cuvier. 



Claws three; no bony nasal septum; ventral scutes osseous; 



supratemporal foramen open Jacare Gray. 



Claws three ; no bony septum : ventral scutes ossified ; supra- 

 temporal foramen roofed over Caiman Spix. 



Claws two ; no bony septum ; ventral scutes ossified (roof over 

 inferior canine wanting on one side) PerosMc/iws Cope. 



The genera Thoracosaurus and Holops are restricted, with present 

 knowledge, to the Upper Cretaceous formations of North America. 

 The other genera still exist, and are restricted to the warmer regions 

 of the earth. Tomistoma is Malaysian; CrocodilKs is cosmopolitan; 

 Osteolccmus is African, while the other genera are South American, 

 excei^t Alligator. This genus includes one North American and one 

 east Asiatic species and one of unknown habitat. Gavialis is Indian. 



In North America we have to do with the widely distributed Alligator 

 mississippiensis and the Crocodilus americanus, which ranges to the 

 southern part of the peninsula of Florida from an extensive habitat in 

 tropical America. 



Many remarkable peculiarities characterize the osteology of this 

 family. 



The pterj'goids have a veiy peculiar form. They are much expanded 

 transversely posteriorly, resembling in this respect tlieCotylosauria and 

 some of tlie Theromora of the Permian, but with this the resemblance 

 ceases. Near the middle line of the skull each one directs its free 

 posterior margin upward, assuming the direction of the basicranial 

 axis and, surrouiuling the posterior nares behind, unites with the 

 inferior descendihg axis of the sphenoid by a transverse serrate suture. 

 The pterygoid plate does not continue its vertical direction for long, but 

 soon unites with the quadrate posteriorly'^ and the alisphenoid ante- 

 riorly by a serrate suture. This union of the pterygoid with the prox- 

 imal extremity of the quadrate, instead of the distal, is a specialization 

 peculiar to this family among reptiles. The superior prolongation of the 



J 



