158 



VERTEBRATE SKELETON 



Ophidia. — The snake skull is somewhat reduced, but still shows 

 plainly Squamate characters, though lacking a parietal foramen. 



The history of the chondrocranium is largely 

 unknown, all studies of it having been made 

 before the introduction of modern methods. 

 A brief statement of the little known follows: 



In the early stage (fig. i68) the notochord nearly 

 reaches the hypophysis, later its anterior end disap- 

 pears, its tip now lying between the otic capsules, 

 while the basal plate, with a basicranial fenestra, is 

 united to the otic capsules, and the metotic pillar is 

 connected dorsally with the capsule which is larger 

 than in most lizards. Its medial and lateral walls 

 chondrify late, the ninth nerve passing through the 

 median wall together with the perilymph duct. The 

 trabeculae, separate for most of their length, are 

 close together in front of the hypophysis, and their 

 anterior ends He in front of the cerebrum and give 

 rise to a low interorbital septum, which supports 

 dorsally a pair of supraseptal plates, which, unlike 

 the septum, chondrify. The rest of the history 

 has not been followed. The nasal capsule is formed 

 by nasal septum, trabecular cornu and a dorsal 

 enclosing plate. The visceral skeleton consists of 

 mandibular and hyoid arches, no branchials having 

 been seen. The pterygoid is simple and lacks 

 processes. The columella arises from the upper 

 end of the hyoid, the rest of the arch separating and 

 wandering farther back. 



The adult skull, which is well ossified, has fewer bones than other 

 Squamates, the result of both fusion and. failure to develop. The 

 cranial cavity (fig. 169), extending to the orbital region, is closed in 

 front by downgrowths from frontals and parietals which meet well 

 ossified ah- and orbitosphenoids. The small premaxillaries bear 

 teeth only in the larger serpents. The maxillae are moderate and, 

 except in poisonous species, bear numerous teeth. In venomous 

 species the maxilla is reduced to a base for the single poison fang, 

 and is so hinged that it folds back on the roof of the mouth when 

 the mouth is closed, and erects when the mouth is opened. The 

 mechanism involved in this includes quadrate, pterygoid and transver- 

 sum, which slide forwards, turning the maxilla on its hinge (fig. 170). 

 Zygomatic and quadratojugal bones are lost as are both temporal 



Fig. 168. — Chondrocra- 

 nium of TropidonotHs 

 (Parker, '79). ac, anterior 

 semicircular canal; c, car- 

 otid canal; fb, basicranial 

 fenestra; //;, hypophysial 

 fenestra; Ic, lateral canal; 

 nc, nasal capsule; pc, pos- 

 terior canal; si, synotic 

 tectum. 



