SKELETON. 



77 



The pterygoquadrate and the Meckelian cartilages bear teeth and form the 

 functional jaws. Most species are hyostylic (p. 73), the pterygoquadrate being 

 supported in front of the orbit by a ethmopalatine ligament on either side; behind 

 by ligament and by the hyomandibular. The Notidanids are amphistylic, the 

 hyomandibular being connected with the rest of the hyoid and not acting as a 

 suspensor of the jaws, but the pterygoquadrate bears a strong process which ar- 

 ticulates with the postorbital process of the cranium. A third condition is found 

 in the holocephalans where the pterygoquadrate, free in the young, becomes auto- 

 stylic by fusion with the cranium. 



The variations in the branchial skeleton (figs. 63, 64) are readily reducible to 

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



FIG. 75. Skull oiSquatina, after Gegenbaur. h, hyoid; hm, hyomandibular; i 2 .1 ', labial 

 cartilages; m, Meckel's cartilage; pq, pterygoquadrate; r, rostrum. 



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 inter- 

 grade that no sharp lines can be drawn. The chondrocranium persists to a consid- 

 erable 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) are 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 that covers the external openings of the gill slits. This is in part 

 of membrane bones, in part of cartilage or cartilage bones. There are two parts 

 to the opercular fold, a gill cover 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, at most, four of these: a preoperculum in front, 

 and behind this in a row from above downward, operculare, suboperculum and 

 interoperculum. The preoperculum overlies hyomandibular, symplectic and 

 quadrate, and it is possible that the opercular bones have been developed in con- 



