450 CROCODILIA 



transfer the choanae further back, owing to the formation of 

 a solid secondary roofing in of the mouth, to transform the 

 amphicoelous into procoelous vertebrae, to reduce the supra- 

 temporal foramina, and to obtain a better development of the dorsal 

 armour, whilst that on the ventral side is gradually reduced. 

 Lastly, there is a tendency towards a shortening and broadening 

 of the snout, a condition which has reached its culmination in 

 the Alligators, while the Gavials are survivals of another branch. 

 The notches in the premaxilla, for the reception of some^ of the 

 lower teeth, have also been acquired independently. Although 

 the recent Crocodilia cannot now, as has been pointed out by 

 Boulenger, be separated into different families, no valid diagnoses 

 being possible owing to the existence of Tomistoma, their phylo- 

 geny shows them to belong to at least two heterogeneous groups. 



KEY TO THE GENERA OF RECENT CROCODILIA. 



I. Snout very long and slender. The mandibular symphysis extends at 



least to the fifteenth tooth, and is partly formed by the splenial bones. 



a. Nasal bones very small, and widely separated from the premaxil- 



laries ..... Gavialis gangeticus, p. 451. 



b. Nasal Tjones long, in contact with the premaxillaries. 



Tomistoma schlegeli, p. 453. 



II. Snout not slender, but triangular or rounded off. The mandibular sym- 



physis does not reach beyond the eighth tooth, and does not reach the 

 splenial bones. 

 a. Fourth mandibular tooth fitting into a notch in the upper jaw. 



1. Without a bony nasal septum . . Crocodilus, p. 454. 



2. Nasal bones dividing the nasal groove. Osteolaemus, p. 466. 

 6. Fourth mandibular tooth fitting into a pit in the upper jaw. 



1. Without a bony nasal septum . . Caiman, p. 471. 



2. Nasal bones dividing the nasal groove . Alligator, p. 466. 



Fam. 1. Teleosauridae, in the Lias and Oolite of Europe ; 

 marine. Snout very long and slender. Nasals widely separated 

 from the premaxillae by the maxillaries. Choanae at the pos- 

 terior end of the palatines. In front of the eye a small 

 sub-lacrymal foramen. Supratemporal foramina large. Vertebrae 

 amphicoelous. Anterior limbs scarcely half as long as the 

 posterior pair. The dermal armour consists of two rows of broad 

 scutes on the back, while the belly is protected by a shield of 

 numerous bony scutes, which are connected with each other by 

 sutures. Teeth numerous and rather slender. General appear- 

 ance like that of Gavials. 



