434 CROCODILIA CHAP. 



vertebrae are devoid of ribs ; the cervicals and first thoracics 

 carry separate capitular and tubercular processes for the attach- 

 ment of the ribs, while the ribs of the rest of the trunk are 

 carried entirely by the long diapophyses, as in the modern 

 Crocodiles. The dermal armour consists of two rows of broad, 

 dorsal, and several rows of smaller, lateral, bony plates. 



Belodon is by far the best-known genus, with several species 

 in South Germany and North America, some of which reached 

 a length of 10 feet, without ventral armour. The closely allied 

 Stagonolepis of the Elgin sandstone in Scotland had dorsal 

 and ventral armour. Other genera in the Triassic formations of 

 India and North America. 



Order III. EUSUCHIA. 



Crocodilia in the stricter sense. The premaxillae are short 

 and always enclose the nostrils. The choanae lie behind the pala- 

 tines, in recent forms even within the pterygoids. They occur 

 from the Liassic or Lower Jurassic period to the present time. 



The direct ancestors of the Eusuchia are still unknown. 

 They cannot have been developed from the Pseudosuchia, nor do 

 we know intermediate stages which connect them with the 

 Parasuchia. The nostrils, situated within the premaxillaries, 

 always lie in front of the nasals, altliough these sometimes 

 extend forwards and form a bony internasal septum fusing w4tli 

 the usual cartilaginous septum. The choanae, instead of opening 

 immediately behind the vomer, are carried far back, owing to the 

 formation of a secondary bony palate. In the Jurassic Crocodiles 

 this roof is formed by the meeting of the palatine bones in the 

 medio-ventral line, and the choanae open immediately behind. 

 From Cretaceous times onwards this roofing is continued by the 

 pterygoids, which likewise form a median suture ; and the united 

 choanae (which may, or may not, be divided by a thin bony 

 septum) are pushed towards the posterior end of the pterygoids. 

 Since the Jurassic times there exists also a tendency to enclose the 

 Eustachian passages (the remnants of the first gill-clefts) by bone. 

 In the earlier members they were still wide slits or open grooves 

 on the ventral side of the basi-occipital bone. Since the Cretaceous 

 epoch they have been transformed into bony canals and open 

 through one median hole, situated between the basi-occipital and the 



