Q2 VERTEBRATE SKELETON 



union of pterotic and squamosal, sphenotic and postfrontal, dermal 

 ethmoid with mesethmoid, etc. 



The chondrocranium (fig. 96) is usually well developed and a large 

 part of the cartilage persists in the adult. In the lower groups (Gan- 

 oids and most Physostomi) the cranial cavity extends into the eth- 

 moid region (platybasic skulls) . In others (tropibasic) the cranium 

 is constricted between the orbits and an interorbital septum (per- 

 manently cartilage or ossifying later) usually occurs, the brain being 

 restricted to the postorbital part of the cranium (fig. 97). 



■piQ_ 97. — Dorsal and side views of chondrocranium of Synguathus fiisciis (Kindred, 

 •21). c, copula communis; ch, ceratohyoid; ct, common trabecula; e, ethmoid plate; 

 ee, ectethmoid; hh, hypohyal; hm, hyomandibula; m, Meckelian; oc, otic capsule; pi, 

 palatine; pq, pterygoquadrate; re, rostral process of ethmoid; sy, symplectic; i, 

 trabecula; tm, median tectum. 



As there are numerous roofing bones, the chondrocranial tegmen 

 is usually more incomplete than in Elasmobranchs, but the synotic 

 tectum usually persists, and not infrequently parts of the cartilage 

 roof are retained farther forwards, forming a transverse bar or 

 plate in the epiphysial region (fig. 96). Most Teleostomes have the 

 cartilaginous otic capsules widely open to the brain chamber. 



The following ossifications of the chondrocranium are the more 

 usual, but some of these are lacking here and there. In the base 



