146 



VERTEBRATE SKELETON 



of which is the common trabecula, expanded dorsally into an imperforate 

 interorbital septum. The supraseptal plates have a regular curve and are 

 perforated by a moderate optic foramen, behind which is a foramen for the 

 ophthalmic artery. The prootic fissure is incomplete, no extension of the 

 sphenolateral reaching from the preotic pillar to the otic capsule. In Chelydra 

 a median parietal taenia connects the pillar with the supraseptal plate; Emys 

 has none. The nasal capsules are nearly entire, the only openings being the 

 nares, looking forwards, a fenestra olfactoria on the medial side and a basal 

 fenestra on the posterior basal surface. 



The pterygoquadrate cartilage, separate from the cranium, is divided in 

 front into epipterygoid and pterygoid processes, while the quadrate shows the 

 features of the adult, being expanded into a vesicle which contains part of the 

 tvmpanum and is traversed by the columella. The hyobranchial skeleton is 

 much reduced, with a rudimentary hyoid and two branchial arches, the hyoid 



Fig. 155. — Dorsal and ventral sides of cranium oiChelone mydas. feo, basioccipital; 

 bs, basisphenoid; /, frontal; tn, maxilla; o, opisthotic; p, premaxilla; pa, parietal; po, 

 postf rental; prefrontal; pi. pterygoid; q. quadrate; s, supraoccipital; sq, squamosal; 

 V, vomer; s, zygomatic. 



copula extending back to the second branchial arch. The hyoid cornu is very 

 short and is separate from the body in later stages, as is the first branchial arch 

 in all known stages. In the cartilage stage the second branchial is continuous 

 with the hyoid corpus. 



The skull of adult Chelonians is compact and (especially in the 

 marine species) suggests that of Theromorphs, although the 

 resemblances are superficial and secondary. Among the general 

 features are the single median naris, orbits entirely surrounded by 

 bone, absence of supratemporal and postfrontal bones and parietal 

 foramen. The base of the cranium has all four occipitalia, the basi- 



