SKELETON 



85 



The variations in the branchial skeleton (figs. 71,72) are readily reducible to 

 the typical conditions. In living elasmobranchs the number of gill arches is five, 

 except in Hexanchus and Chlamydoselache (six) and Heptanchus (seven). Hyoid 

 and branchial arches bear numerous branchial rays which support the gills and 

 the gill septa, while smaller cartilages on the inner surface of each arch extend 

 into the gill strainers. 



TELEOSTOMES show a wide range of structure of skull, yet the series so 

 intergrade that no sharp lines can be drawn. The chondrocranium persists to a 

 considerable extent, and numerous membrane bones are present, supplementing 

 those of cartilaginous origin. With few exceptions cartilage bones (the four 

 occipitals, orbito- and alisphenoids and prootics are the most constant) 



Fig. 85. — Side view of skull of mackerel (Scomber) after Allis. ar, articulate; as, 

 alisphenoid; bo, basioccipital; d, dentary; enp, entopterygoid; eo, exoccipital; ep, ectop- 

 terygoid; es, extrascapular; epo, epiotic; /, frontal; to, interopercular; etk, ethmoid; /, 

 lacrimal; mx, maxillary; mxp, metapterygoid; n, nasal; op, opercular; p, parietal; 

 pe, petrosal; />/, postfrontal; pi, palatine; pm, premaxillary; po, preoperculum; poo, 

 postorbital; prf, prefrontal; ps, parasphenoid; q, quadrate; sbo, suborbital; so, supraoc- 

 cipitai; sop, suboperculum; spo, sphenotic; sq, squamosal; ssc, suprascapular: sy, 

 symplectic. 



arc developed, while the inner wall of the otic capsule disappears, so that the 



cavity is connected with that for the brain. Even more characteristic is the 



presence of skeletal structures supporting the opercular fold which covers the 



external openings of the gill slits. This is in part of membrane bones, in part of 



irtilage or cartilage bones. There are two parts to the opercular fold, a gill 



ver or operculum above and a branchiostegal membrane below. The latter 



is supported by branchiostegal rays, comparable to the hyoid branchial rays of 



the elasmobranchs, while the operculum contains membrane bones, there being 



most, four of these: (fig. 85) a preoperculum in front, and behind this in a 



row from above downward, operculare, suboperculimi and interoperculimi. 



The preoperculum overlies hyomandibular, sympleclic and quadrate, and it is 



