156 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the dorsal aspect of the. buccal mass (Fig. 34) we see posteriorly a dark semicircle, the posterior 

 edge of the inner flange of the upper jaw covered by only a thin membrane. From the entire 

 outer surface of the inner flange of the upper jaw muscle bundles extend to the inner surface of 

 the outer Mange and out between the flanges of the lower jaw, the mandibular muscle. This 

 powerful muscle, which occupies the entire space between these flanges of the two jaws, forms 

 the closing muscle of the jaws. When we see this we are no longer surprised at the Nautilus's 

 ability to nip off the leg of a chicken as if with a pair of shears. A groove along the mid-dorsal 

 line indicates the line of nearly complete separation of the muscle bundles of the right and left 

 sides. The separation is not complete, for many bundles cross from each side to the other. An 

 artery runs immediately above this groove, but outside the muscular membrane, the branches of 

 which are distributed to the mandibular muscles. 



Upon the ventral side of the buccal mass a much more complicated arrangement holds. 

 (Fig. 33.) Centrally the radular sac may be seen extending more than half the length of the 

 buccal mass. At the sides are muscles which control some of the mouth parts, and portions of 

 the buccal nervous system. I find, by comparing my notes and figures, that these muscles are 

 slightly variable, therefore I shall describe only such as seem to be fairly constant. 



By reference to Fig. 33 it will be seen that several muscles on each side have their origins 

 in the muscular membrane already described as covering the posterior part of the buccal mass. 

 Two pairs of these muscles (1 and 2) pass into the cavity of the anterior of the two folds in front 

 of the tongue (the anterior prelingual process) and are attached to its walls, evidently being 

 retractors of the organ. The median pair (2) unite as they enter the fold, but after their union 

 give off several small branches. 



A third pair of muscles (4) arise near the side of the buccal mass and run upward and inward 

 to within the fold immediately in front of the tongue (the posterior prelingual process), but 

 mostly to the fold which bears the anterior free part of the radula. A peculiar muscle (figured 

 in Fig. 33, 3) arises with a double head from the muscular membrane closing the front of the 

 space between the ventral buccal retractors and runs forward, at first above and then beneath 

 the radular sac, to be attached to the fold in front of the tongue. Two pairs of slender muscles 

 (5 and t!) arise from the posterior part of the muscular membrane. The longer pair (5) extend 

 forward under the muscles already described to the surface of the tongue bearing the free part of 

 the radula. The shorter pair of muscles extend only to the membrane covering the slight 

 downward projection of the upper jaw. 



With muscles 1 and 2 arises a third quite strong muscle (T), which passes upward and directly 

 inward to be attached to the dorsal surface of the radular sac. From its position I should think 

 that this muscle may function either as protractor or retractor of the radular sac, always tending 

 to pull it back to its resting position. 



From each ventral posterior corner of the inner flange of the upper jaw a strong muscle (8) 

 runs forward and outward to be attached to the ventral edge of the outer flange of the lower 

 jaw. The contraction of these muscles probably acts to open the jaws. The central part of the 

 buccal nervous system lies beneath these muscles; the pharyngeal commissure, however, runs 

 above them, between them and the lateral parts of the mandibular muscles. 



Immediately above the posterior portion of the radular sac is ti muscular membrane, containing 

 both transverse and longitudinal fibres, which forms the ventral wall of the space containing the 

 lingual and radular muscles. Quite a number of small muscles extend between this membrane 

 and the dorsal wall of the radular sac. while the membrane itself is attached anteriorly to the 

 radular sac. From the dorsal side of the membrane and the posterior side of the anterior 

 upwardly directed portion of the radular sac a median septum extends to the dorsal integument 

 of the tongue. (Fig. 35, S.) This septum, thin ventrally, becomes thicker and muscular dor- 

 sally, and is split into two parts, which are attached to the bottoms of the V-shaped folds extend- 

 ing back from the opening of the radula. (Fig. 35, X.) 



Upon each side of the septum are three muscles which form the mass of the tongue, and 

 arc chief in controlling the movements of the tongue and radula. Next to the septum is a 

 broad, flat muscle (the internal lingual, IL), which runs parallel to the septum, to be inserted 



