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 
“tf 
le 
Idm ly eb 
Fic. 73.—Ventral and lateral views of the skull of lamprey (Petromyzon marinus), after 
Parker. ad, anterior dorsal cartilage, bb, branchial basket; gc, gill cleft; Jc, labial carti- 
lage; Jdm, lateral distal mandibular; /g, lingual cartilage; nc, nasal capsule; oc, otic capsule; 
on, optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage. 
ec ds Pg ho Br’ bre b 
Fic. 74.—Side view of cranium of Bdellostoma, after Ayers and Jackson. 5, basal 
plate; br, branchial basket; c, cornual cartilage; d, dental plate; #, hyoid; J, lateral labial 
cartilage; x, nasal tube; nc, notochord; 9, otic capsule, oc, olfactory capsule; pg, 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, 
i 
