XIII 



PHYLUM CHORDATA 



353 



oecLUs also in most 



ramus plays, an arrangement which 

 Lacertilia. 



In accordance with their purely aerial mode of respiration, the 

 xiscerrd an-Jics are much more reduced in the Reptilia than in the 

 Amphibia in general. Tlie only well-ileveloped })ost-mandibular 

 arch is the hyoid, and even this may undergo considerable 

 reduction (Ophidia). The branchial arches, except in so far as 

 they contribute to the formation of the laryngeal cartilages, are not 

 represented in the adult, with the exception of most Chelonia, in 

 which the first and second branchial 

 arches persist as cornua of the hyoid. 



There is little variation in the 

 structure of the limb-arches and 

 skeleton of the limbs in the diif'erent 

 groups of Lacertilia. The pelvic 

 arch is ilistinguished in the 

 Lacertilia in general by its slender 

 character ; and the pubes and 

 ischia are, as in fact is the case 

 throughout the class, separated from 

 one another by wide ischio-pubic 

 foramina — a feature which markedly 

 distinguishes the leptilian pelvis 

 from that of the Amphibia. Tn 

 limbless forms the pectoral arch 

 may be present or may be absent. 

 Li the Ophidia all trace of limbs is 

 as a rule, absent ; but in some 

 Pythons vestiges of hind-limbs are 

 to be detected in the form of two 

 or three small bones which support 

 a small horny claw. 



In Sphenodon (Fig. 984) there is 

 a foramen above the outer and one 

 above the inner condyle of the 

 humerus. There are eleven carpal 

 elements, of which there are four, 

 including a pisiform, in the proxi- 

 mal row, two centrals, and five in the distal row. The pubes 

 are united in a symphysis, in front of which is a cartilaginous 

 epipubis. A large oval foramen intervenes between the iscliium 

 and the pubis. A cartilaginous hypo-iscliium is attached to the 

 ischia behind. Li the tarsus the tibial and fibular elements 

 are distinct, though firmly united. The intermedium and the 

 centrale are firmly fixed to the tiblale. There are three distal 

 tarsal bones. 



In the Chelonia (Fig. U86) the interclavicle (episternum) and 



Coi'c 



Fig. 994. — Veutml view of the skull of 

 young Crocodile. Ch, posterior 

 naves ; Voce, occipital condyle ; Jij. 

 jugal ; M. maxilla (ij.alatine process ) ; 

 Oil. basioccipital ; Orb. orbit ; PI. 

 palatine ; Pmx. premaxillas ; Pt. 

 pterygoid ; Qj. quadrato-jugal ; Qu. 

 quadrate. (From Wiedershcim's Com,- 

 Itarativc Anatoiwi.) 



