Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 199 



tendon. This tendon is inserted by two heads, one on each side, on 

 the inner surface of the dentary close to the symphysis. 



In the median Hne of the ventral surface of the united 

 superior geniohyoidei of oppposite sides of the head, there 

 is a longitudinal depression, or concavity, in which lie the 

 almost completely fused superficial, inferior portions of the two 

 muscles. These inferior muscles (GJii) take their origins from 

 the deeper, superior muscles, and thus are partly fused with 

 those muscles as well as being almost completely fused with each 

 other. Together they form a single muscle, which is nearly 

 round in section and runs directly forward in the middle line of 

 the head. On the ventral surface of this muscle there is usually 

 a slight median groove, which marks the line of separation of 

 its two component parts ; and, between it and the superior part 

 of the muscle, on each side, there is a lateral and deeper groove. 

 Anteriorly, the median, ventral groove disappears, and the ventral 

 fibers of the inferior muscle unite and are inserted, either directly, 

 or by a fiat, median tendon, on the inner surface of the mandible, 

 at the symphysis. The dorsal fibers of the two muscles, on the 

 contrary, separate at this point completely, not only from each 

 other but also from the deeper, superior muscle, and form two 

 round and tapering muscle bundles, each of which is inserted, 

 by a relatively long and small tendon, on the inner surface of the 

 ramus of the mandible of its own side, near the symphysis. All 

 three tendons of the inferior muscles lie ventral to the interman- 

 dibularis, the single, double-headed tendon of the superior mus- 

 cle lying dorsal to it. The median tendon of the inferior mus- 

 cles has the most ventral insertion, the insertion of the tendons of 

 the superior muscles is next dorsal to it, while the two tendons 

 of the inferior muscles pass dorsal to the tendons of the superior 

 muscles, along their lateral edges, and are there inserted. Dor- 

 sal to the insertion of the latter tendons the mucous lining mem- 

 brane of the mouth turns backward and then forward again, thus 

 forming a delicate fold with a free posterior edge. The fold is 

 narrow but long, extending a considerable distance backward, on 

 each side, along the inner edge of the mandible, and is the man- 

 dibular breathing valve described by Dahlgren (No. 21) in other 

 teleosts. 



