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 five, 
Fic. 75.—Skull of Squatina, after Gegenbaur. h, hyoid; km, hyomandibular; 1” .1', labial 
cartilages; m, Meckel’s cartilage; pg, pterygoquadrate; 7, 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- 
