18 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



in Figure 16, 1. rot. I. Its function evidently is to rotate the mandible 

 in such a way as to raise the molar surface. The opposing muscle is 

 designated as 3. ret, rot. 



2. Abductor. An extensive muscle (Fig. 14, 2. abd.), which arises at 

 the side of and anterior to the lateral rotator, passes obliquely forward 

 and downward, and is inserted obliquely on the lateral surface of the 

 mandible, which is excavated to receive the belly of the muscle. This 

 is the only muscle which can act in opposition to the adductor (9. add.), 

 and may also assist in the process of retraction. 



3. detractor and rotator. A slender muscle (Fig. 14, 3. ret. rot.), 

 which originates near the base of the skull in a dorso-lateral position, 

 passes forward and downward, and is attached to the inner side of the 

 basal ridge of the mandible described above. 



4. Retractor. Although distinct from the preceding retractor, it 

 follows nearly the same course (Fig. 14, 4- ifet.), but is more ventral; it 

 originates nearer the median plane, and is inserted on the basal ridge 

 immediately behind its companion. Both these muscles are adapted to 

 withdraw the mandible into its socket after it has been protruded by 

 muscles 5 and 6 ; No. 3, however, appears in addition to be the only 

 muscle which is capable of rotating the mandible in opposition to Nos. 

 1, 7, 8, and 10, that is, so as to lower the molar surface. 



5. Lateral Protrusor. The contraction of the outer of two slender 

 cylindrical muscles (Fig. 14, 5. prH. I.) which originate on the anterior 

 arm of the tentorium results in protruding the mandibles. It passes 

 upward and backward from the tentorium along the mesal surface of 

 the mandible, and is inserted immediately under the insertion of Xo. 3. 

 Its function is doubtless to protrude the mandible by pulling its base 

 forward. 



6. Mesal Protrusor. This muscle accompanies No. 5 (Fig. 14, 6. pr't. 

 ms.), at the side of which it originates, but its course is more ventral, 

 and its insertion is on the basal ridge just behind that of No. 5. The 

 last two muscles are distinct, but have the same function, that of pro- 

 truding the mandibles in opposition to muscles 3 and 4. 



7. Rotator. A long stout muscle (Fig. 14, 7. rot.), which begins on 

 the side of a median dorsal chitinous projection near the base of the 

 head. The muscle runs forward, outward, and downward, and terminates 

 in a tapering pigmented tendon, which crosses under the base of the 

 mandible (Fig. 15, 7. rot.) and is inserted in the outer angle of the 

 large triangular opening. 



8. Rotator. A long, powerful muscle, which originates near the base 



