22 



lower surface of the fleshy mass to be inserted into the base of the under mandible 

 so as evidently to depress it. 



The circular Up (m. pi. 4 ; c. fig. 5. pi. 8.) immediately surrounding the jaws, is, 

 in consequence of the greater extent of the outer plate of the under mandible, 

 much deeper than in the Cuttle-fish. It is composed of two layers of thin but 

 tough membrane, terminating anteriorly in numerous minute pointed and jagged 

 processes forming a fringe, and well adapted to entangle and prevent the loss of 

 the smaller fragments of the food while it is undergoing the comminuting action 

 of the mandibles. This hp is thickened towards the margin by the interposition 

 of muscular fibres, the exterior of which form between the two layers of mem- 

 brane a thin circular sphincter, while the inner fibres are longitudinal, and 

 extend nearly to the base of the mandibles ; so that all the reqmsite motions of 

 this part are adequately provided for. The outer layer of the hp is continued 

 upon the labial processes, and parts immediately exterior ; the inner layer turns 

 in beneath the outer lamina of the mandibles, and is closely attached to the 

 muscular basis. 



This apparatus of the jaws is provided with four retractor muscles, and one 

 for protrusion. Of the former, two are superior and two inferior. The upper 

 pair (q. q. pi. 4.) arise from the extremities of the cephalic cartilages, and con- 

 verge to be inserted in the groove between the outer and inner plate of the upper 

 mandible. The lower pair {g. fig. 5. pi. 8.) come off from the body of the skeleton 

 at the base of the inferior labial processes ; after a short course they expand into 

 a kind of pouch, supporting the pharynx, and are inserted along the inferior 

 margin of the lower mandible. The jaws are protruded by a strong semicircular 

 muscle (r. pi. 4; k. pi. 7.), which is continued from the margin of one inferior 

 labial process over the mandibles and their retractor muscles to the labial pro- 

 cess of the opposite side. In this action the muscle is probably assisted by the 

 inner circular fibres of the oral sheath. 



The tongue of Nautilus (fig. 6 & 7. pi. 8.) is a beautifully constructed part. It 

 is of large size, filling the whole cavity comprehended between the rami of the 

 inferior mandible. It is supported by an oblong horny substance, about eight 

 lines in length, shghtly curved, and transversely striated, which may be con- 

 sidered as representing an os hyoides (fig. 6. pi. 8.). The posterior extremity of 

 this substance is free, or connected only by a few filaments with the parts above j 



