isi'"^] Davidson: The Musculature of Heptanchus maculatus 161 



is iuternally in a groove on the distal third of the epibranehial segment 

 and they are inserted similarly on the proximal part of the cerato- 

 branchial segment of each arch. Their contraction flattens the 

 branchial region. 



V. Hypobranchials (fig. 4). The hvpobranchial or ventral longi- 

 tudinal muscles lie on the ventral side anterior to the pectoral girdle. 

 They are thick and solid and their shape is correlated with the shape 

 of the body. They include the eoracoarcuales communes, a pair of 

 muscles extending forward from the pectoral girdle halfway to the 

 mandibular symphysis; the coracomandibularis, a median ventral 

 unpaired muscle ; the coracohyoideus, a pair apparently continuous 

 anteriorly with the coracoarculaes ; and the coracobranchiales, includ- 

 ing seven pairs of flat muscles extending to the gill-arches. This 

 group of muscles has its origin on the coracoid portion of the scapula 

 and from the connective tissue forming the floor of the pericardial 

 cavity which fascia is attached to the coracoid. 



1. Coracoarcuales communes {car.). The eoracoarcuales muscles 

 lie directly under the integument just anterior to the pectoral girdle. 

 At their origin they are quite broad but they rapidly decrease in size 

 anteriorly. Several myosepta may be seen to cross each of these 

 muscles and give to them the same appearance as the musculature of 

 the body. Their origin is from the anterior surface of the coracoid. 

 The median fibers are inserted in the strong membrane which forms 

 the floor of the pericardial cavity, while the lateral fibers are con- 

 tinued forward and are inserted in the fascia in M^hich the cora- 

 cohyoideus muscles have their origin. 



2. Coracomandibularis (c.md.). The coracomandibularis is an 

 unpaired muscle lying dorsal to the first and second ventral con- 

 strictors. Its origin is in the fascia dorsal to and between the anterior 

 part of the coracoarcuales. At its origin it is laterally compressed, 

 but immediately becomes rounded and is inserted ventrally on the 

 posterior edge of the mandible, extending on either side of the 

 symphysis. 



3. Coracohyoideus (c. Jiy.). The coracohyoideus muscles are the 

 largest of the ventral longitudinal group. They are paired and are 

 just dorsal to the coracomandibularis. The coracohyoideus muscles 

 are a direct anterior continuation of the coracoarcuales. Their origin 

 is from the fascia in which the latter insert. The more dorsal fibers 

 take origin in the fascia between the coracohyoideus and coraco- 

 branchiales. The muscles are broad and thick and uniform in size 



