184 Thirty-fifth Keport ok the State Museum. 



gin, the base a little above a median line ; it is oval in shape, about 

 one-third longer than wide. 



The posterior adductor (PI. 8, p. a.) muscle is situated on a line with 

 the anterior, a little more distant from the posterior margin than the 

 anterior muscle is from the anterior margin ; it is one and two-thirds 

 larger than the anterior muscle, about one-third longer than wide. 

 The protractor pedis muscle is situated just posterior to the base of 

 the anterior adduptor muscle (PI. 8, p.p.); it is fan-shaped in appear- 

 ance, spreading over a large portion of the body and foot (PL S, fig. 2, 

 p.p.), very near the surface. If the body is scraped gently with a 

 knife the strong muscular fibres will be immediately seen. This mus- 

 cle acts in opposition to the anterior and posterior retractor. 



The anterior retractor muscle (PL 8, fig. 1, a. r.) has its origin just 

 posteriorly to the upper part of the anterior adductor muscle. The 

 impression made by the attachment of the muscle to the shell being 

 about one-third the size of that of the anterior adductor. The greater 

 portion of the fibres of this muscle pass downward toward the ven- 

 tral and anterior margin of the foot, having their greatest develop- 

 ment in the anterior portion, but some of the fibres pass upward, over 

 and through the substance of the liver. The fibres are for the most part 

 more deeply imbedded than either the protractor pedis or posterior re- 

 tractor fibres. 



The posterior retractor muscles have their origin jnst anterior to 

 the upper portion of the posterior adductor (PL 8, fig. 3, p. r.). The 

 impression made by the attachment of this muscle to the shell is about 

 one-eighth the size of that of the posterior adductor muscle. It is at 

 first a muscular stem, but soon spreads, continuing through the lower 

 portion of the body and the foot, finding its greatest development in 

 the foot. Some of the fibres pass among those of the protractor 

 pedis, but as a rule they are beneath them. 



At a short distance from the ventral margin the mantle is attached 

 to the shell by numerous small muscles along a line essentially parallel 

 with the margin of the shell ; the impression of these muscles form- 

 ing the pallial line. Their fibres extend to the ventral margin of the 

 mantle, composing a large portion of that part of the mantle. 



If a portion of the muscle is examined under the microscope it will 

 be seen that the fibres are composed of spindle-shaped bands (PL 4, 

 fig. 9), each of which contains an elongated nucleus. The space 

 around the nucleus is clear, but the rest of the band contains a great 

 number of granules, arranged in somewhat indistinct transverse rows. 



