224 HYDROSAURIA. 



roof for the buccal cavity. The posterior nares which are surrounded 

 by the paired vomers open at the posterior margin of the buccal 

 cavity. The conical teeth, which are completely confined to the 

 bones of the jaws [prsemaxillse, maxillae, and mandible], are deeply 

 wedged into alveoli, and they present slightly compressed striated 

 crowns. The fourth tooth of the mandible is usually distinguished 

 by its great size as a prehensile tooth, and, when the jaws are shut, 

 fits into a gap or an excavation in the upper jaw. In the Teleo- 

 sauria the vertebras are amphiccelous ; in the Steneosauria, which are 

 also extinct, they are opisthoccelous, and in the Crocodiles of the pre- 

 sent day proccelous. 



The internal organisation of the living Crocodiles is the highest 

 amongst all Reptiles. The eyes have vertical pupils and two lids as 

 well as a nictitating membrane. The nasal openings lie far forward 

 on the point of the snout, and, as well as the ears which are placed 

 far back, can be closed by cutaneous valves. The buccal cavity, to 

 the floor of which is attached a flat non-protractile tongue, is without 

 salivary glands, and leads by a wide cesophagus into the rounded 

 muscular stomach, which resembles that of Birds in form and struc- 

 ture, and specially in the aponeurotic discs of its internal lining. 

 The stomach is followed by a thin-walled duodenum,- which is beset 

 with papillae, and passes into the small intestine, which is folded in 

 a zigzag fashion. There is no csecal appendage to the short wide 

 large intestine. The latter becomes narrow and almost funnel- 

 shaped, before it opens into the cloaca, from the anterior wall of 

 which arises the erectile copulatory organ. The structure of the 

 heart is the most perfect found in all Reptiles, and, in the com- 

 plete separation of a right venous and a left arterial portion, affords 

 a direct transition to that of the warm-blooded animals. Finally, 

 the free communication of the body cavity by openings of the 

 so-called peritoneal canals, which recall the abdominal pores of the 

 Ganoids and Selachians, deserves to be mentioned as peculiarities of 

 the Crocodilia. 



Three groups of Crocodiles are to be distinguished ; two of these 

 the Tdeosauria (AmpMcoelia) and Steneosauria (Opisthoccelia) are 

 extinct. The former with the genera Mystriosaurus Kp. and Teleo- 

 saurus Geoffr. are confined to the Jurassic formation, the latter with 

 Steneosaurus Geoffr. Cetiosaurus Owen, etc., occur in the Jura and 

 in the chalk. Only the third group of the Crocodiles or Proccelia 

 has persisted from the chalk onwards through the tertiary period to 

 our own time. 



