GREGORY: FISH SKULLS 425 



apparently compensated by the forced respiratory current set up by the syringe-like move- 

 ments of the muscular opercular flap. In fishes with heavy pharyngeal dentition the 

 supporting parts of the branchial apparatus are correspondingly strengthened or braced 

 against such near-by bones as the cleithrum (in the scarids) or the base of the cranium 

 (in the clupeoids). 



Muscles of the Branchial Arches and Jaws. — In the sharks, as already noted, there is an 

 elaborate system of muscles for flexing and extending the joints of the orobranchial appa- 

 ratus, including superficial constrictors, which pass over the surface of the arches and the 

 deep extensors and flexors (see Fig. 4). In the teleosts some of the dorsal muscles of the gill- 

 arches spring from the side of the braincase, while the ventral ones form a complex system 

 extending forward from the anterior border of the cleithrum and extending transversely 

 across the floor of the throat between the hyoid arches. The branchial muscles, which 

 have been beautifully figured by AUis in Chlamydoselachus, Amia, Scor,d?er, are doubtless 

 modified in many special ways in accordance with the various movements of the branchial 

 arches, but about this subject little is known. 



The jaw muscles, which are regarded by Vetter (1874; 1878, p. 542) as serially homol- 

 ogous with the middle deep flexors of the branchial arches, extend fan-wise from the curved 

 border of the preopercular to the mandible; one division (Ad 1) being connected with a 

 tendon that is inserted on the medial surface of the maxilla. These muscles have been 

 described in detail especially by Vetter (1874, 1878) and by AUis (1903, 1909). Their 

 tendinous parts meet in a complex central tendon with several branches to the maxilla 

 and mandible. The fan-like origin of A\, A2, from the preopercular and from the area in 

 front of it probably insures smooth, continuous and efficient action as the mandible moves 

 upward and as one part of the adductor after another comes into the phase of maximum 

 extension. As noted above (p. 414) the mandible as a whole acts as a lever of the third 

 class in which the point of application of the power lies between the resistance and the 

 fulcrum. The coronoid process, either of the dentary or of the articular bone, acts as a 

 lever to increase the power and decrease the range of movement of the teeth, especially of 

 those on the front end of the mandible. Hence the coronoid process of fishes with strong- 

 biting or pincers-like jaws is usually large or massive in proportion to the length of the 

 mandible (Figs. 71, 167, 124, 133). 



The Opercular Elements. — As to the history of the gill-covers, it is well known that these 

 are foreshadowed in the opercular flaps of the chimseroids, and that all known earliest 

 Crossopterygii and Actinopterygii were already in possession of a complete or nearly com- 

 plete opercular system. The opercular, subopercular and branchiostegal bones arise as 

 local dermal ossifications in a folded flap of skin that projects backward from the hyoid 

 arch. The opercular is attached by a concave facet to the opercular pedicle of the hyo- 

 mandibular, while the subopercular and branchiostegals are connected by ligaments with 

 the lower segments of the hyoid arch. The interopercular according to Tate Regan (1929, 

 p. 313) represents the lower end of the opercular, but Allis (1909, p. 69) suggests that it may 

 be regarded as the branchiostegal ray of the interhyal, with which it is connected. It is also 

 attached by a short ligament to the angular bone of the mandible, and by watching any 

 large percoid fish in an aquarium one may see that in the respiratory movements the inter- 

 opercular keeps the opercular series in step with the mandible, while the branchiostegals 

 move up and down with the cerato- and epi-hyals. 



