76 COMPARATIVE MORPHOLOGY OF VEGTEBRATES. 



ELASMOBRANCHS have a nearly typical chondrocranium which is never 

 divided into separate elements and is never ossified. The floor is complete, the hypo- 

 physis resting in a sella turcica. Above there is an anterior fontanelle, closed by 

 membrane and a posterior fontanelle may occur. The occipital region typically 



FIG. 73. Ventral and lateral views of the skull of lamprey (Petromyzon marinus), after 

 Parker, ad, anterior dorsal cartilage, bb, branchial basket; gc, gill cleft; Ic, labial carti- 

 lage; Idm, lateral distal mandibular; Ig, lingual cartilage; nc, nasal capsule; oc, otic capsule; 

 on, optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage. 



n 



FIG. 74. Side view of cranium of Bdellostoma, after Ayers and Jackson, b, basal 

 plate; br, branchial basket; c, cornual cartilage; d, dental plate; h, hyoid; /, lateral labial 

 cartilage; n, nasal tube; nc, notochord; o, otic capsule, oc, olfactory capsule; pq, pterygo- 

 quadrate bar; sp, suprapharyngeal plate. 



articulates with the vertebral column by a pair of prominences, the occipital con- 

 dyles, but in most species this joint is not functional, the skull being immovably 

 united to the backbone. In front the snout is supported by rostral cartilages, 

 usually three in number, but these are frequently fused to a single mass. 



