540 LOUISE SMITH 



The adult geniohyoideus consists of two parts, a more super- 

 ficial geniohyoideus medialis and a deeper one, the geniohyoideus 

 lateralis. The medialis corresponds closely to the entire muscle 

 in the larva. It is a rather broad, extremely thin, ribbon-like 

 band, with its somewhat thicker lateral edge in close association 

 posteriorly with the ceratohyoideus internus which lies just 

 beneath it. As in the larval muscle, its origin is the posterior 

 border of the mandible, lateral to the symphysis, and its insertion, 

 the anterior edge of the os thyreoideum (= distal end second 

 basibranchial) . The lateralis arises with the medialis (fig. 22), 

 but lies dorsal and lateral to it, and is inserted along the entire 

 outer border of the ceratohyal. A very thin sheet of fibers 

 from this portion also passes posteriorly dorsal to the hyoid 

 and is inserted in the mucous membrane of the pharynx behind 

 the posterior end of that cartilage (figs. 24 and 25). 



The thoracicohyoideus, as such, has broken down, but is still 

 partially represented by the sternohyoideus and the two divisions 

 of the abdominohyoideus. The sternohyoideus is a very thin 

 superficial muscle arising from the dorsal surface of the sternum, 

 and inserted on the posterior border of the os thyreoideum. In 

 its course, it passes latero anteriorly, crossed by two myocommata 

 to a raphe behind the anterior end of the procoracoid, whence it 

 continues medio anteriorly to its insertion. Thus the inner edge 

 of the muscle forms, with that of the other side, a rhomboidal 

 space over the pericardium (fig. 19). 



The abdominohyoideus is a very long, round muscle arising 

 on the pelvic girdle and extending forward at the sides of the 

 body, crossed by many myocommata, to a position about behind 

 the most anterior raphe in the sternohyoideus. From this point 

 it sends off a small round slip running anteromedially and 

 inserted on the middorsal surface of the os thyreoideum." The 

 main portion also begins at this point to bend a little medially 

 toward the posterior end of the first basibranchial. There the 

 fibers diverge; some pass straight forward, ventral to the lateral 



^ I find no indication that this slip is continued anteriorly beyond the os 

 thyreoideum as the part of the geniohyoideus medialis which Wiedersheim letters 

 T {'75, fig. 133) and so describes (ibid., pp. 187-8). 



