THE SKULL. 



571 



the lateral walls of the skull. In front there is usually a cleft 

 separating it from the alisphenoid region of the skull, through 

 which the third division of the fifth nerve passes out. This cleft 

 becomes narrowed to a small foramen (fig. 327, V}. The 

 sclerotic cartilage is always free, but profoundly modifies the 

 region of the cranium near which it is placed. The nasal invest- 

 ment forms in Elasmobranchs (fig. 327, No] a capsule open 



PI. ft 



B.B-r 



FIG. 327. SKULL UK ADULT DOGFISH, SIDE VIEW. (From 1'arker.) 

 O.C. occipital condyle ; An. periotic capsule; Pt.O. pterotic ridge ; Sf. O. sphen- 

 otic process ; S. Or. supraorbital ridge ; Na. nasal capsule ; P.N. prenasal cartilage; 

 //. optic foramen; 1'. trigeminal foramen; Pl.Pt., Qu. pterygo-quadrate arcade; 

 M.Pt. metapterygoid ligament (including a small cartilage) ; PI. Tr. ethnic-palatine 

 or palato-trabecular ligament ; Rick, lower jaw ; Sp. spiracle; II. M. hyomandibular; 

 C.Hy. ceratohyal ; m.h.L mandibulo-hyoid ligament; Ph.Br. pharyngobranchial ; 

 E.Br. epibranchial ; C.l: ceratobranchial ; H.Br. hypobranchial ; B.Br. basi- 

 branchial ; Ex.Br. extrabranchial ; 7 1 , ", 3 , 4 , 5 . labial cartilages ; the dotted lines 

 within Mck. indicate the basihyal. 



below, and continuous with the ethmoid region of the trabeculae. 

 In most types however it becomes more closely united with the 

 ethmoid region and the accessory parts belonging to it. 



The cartilaginous cranium, the development of which has 

 been thus briefly traced, persists in the adult without even the 

 addition of membrane bones in the Cyclostomata, Elasmobranchii 

 (fig. 327) and Holocephali. In the Selachioid Ganoids it is also 

 found in the adult, but is covered over by membrane bones. In 

 all other types it is invariably present in the embryo, but becomes 

 in the adult more or less replaced by osseous tissue. 



