366 REPTILIA. 



the animal in turning quickly) ; and there are generally false 

 abdominal ribs produced by the ossification of the tendinous 

 intersections of the recti muscles. There are no clavicles. 

 The heart consists of four completely distinct and separate 

 cavities, two auricles, and two ventricles, the ventricular sep- 

 tum as in no other Reptiles being complete. The right and 

 left aortse, however, or, in other words, the pulmonary artery 

 and systemic aorta, are connected together close to their origin 

 by a small aperture (foramen Panizzcz), so that the two sides 

 of the heart communicate with one another. The aperture of 

 the cloaca is longitudinal, and not transverse, as in the Lizards. 

 All the four limbs are present, the anterior ones being penta- 

 dactylous, the posterior tetradactylous. All are oviparous. 



The chief points by which the Crocodiles are distinguished 

 from their near allies, the Lacertilians, are the possession of 

 a partial bony dermal exoskeleton in addition to the ordinary 

 epidermic covering of scales, the lodgment of the teeth in 

 distinct sockets, and the fact that the mixture of venous and 

 arterial blood, which is so characteristic of Reptiles, takes 

 place, not in the heart itself, but in its immediate neighbour- 

 hood, by a communication between the pulmonary artery and 

 aorta directly after their origin. 



The order Crocodilia is divided into three sub-orders, termed 

 Procxlia, Amphicalia, and Opisthocalia according as the dorsal 

 vertebrae are concave in front, concave at both ends, or con- 

 cave behind. The sub-order PrococKa comprises all the living 

 forms namely, the Crocodiles proper, the Alligators, and the 

 Gavials. The first of these have the fourth tooth in the lower 

 jaw (fig. 317) larger than the others, forming a canine tooth, 



Fig- 317- Skull of young Crocodilns biporcatus v afier Van der Hoven). 



which is received into a notch excavated in the alveolar border 

 of the upper jaw, so that it is visible externally when the mouth 

 is closed. In the Alligators (fig. 318), the fourth tooth in the 

 lower jaw forms a canine which is received into a pit in the 

 palatal surface of the upper jaw, where it is completely con- 

 cealed when the mouth is shut. In the Gavials the snout is 



