xii PHYLUM CHORDATA 443 



the mouth being opened very widely so as to permit of the 

 passage of the relatively large animals which the Snake 

 swallows whole ; this wide opening of the mouth is further 

 aided by the two halves of the mandible not being firmly fixed 

 together anteriorly, but merely connected together by means 

 of elastic tissue, so that they are capable of being separated 

 from one another. 



In accordance with their purely aerial mode of respiration, 

 the visceral arches are much more reduced in the Reptilia than 

 in the Amphibia in general. The only well-developed post- 

 mandibular arch is the hyoid, and even this may undergo 

 considerable reduction (Ophidia). The branchial arches, 

 except in so far as they may contribute to the formation of 

 the tracheal rings, are not represented in the adult, with 

 the exception of most Chelonia, in which the first branchial 

 arch persists. 



In the Lizard (Fig. 252) and Crocodiles there is a cross- 

 shaped membrane-bone, the interclavicle or episternum (epist) 

 in relation to the pectoral arch and sternum. In the limbless 

 Lizards the pectoral arch may be absent or may be well 

 developed : it is completely wanting in all Snakes. In the 

 pelvic arch the ischium is separated from the pubis by a wide 

 space not developed in the Amphibia. 



In the Lizards teeth are present on the premaxillae, the 

 maxillae, the mandible, and usually the palatines : they are 

 in most Lizards small and simple, and uniform in shape 

 and arrangement. A Mexican Lizard, Heloderma, differs 

 from all the rest in having teeth which are perforated for. the 

 ducts of poison-glands. In the Snakes (Fig. 251) teeth are 

 rarely developed on the premaxillse, but are present on the 

 maxillae, palatines, pterygoids, and sometimes the transverse 

 bones extending between the pterygoids and maxillae, as 

 well as on the dentary of the mandible. They may be 



