184 THIRTY-FIFTH REPORT ON 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 (Pl. 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 adductor muscle (Pl. 8, p.p.); it is fan-shaped in appear- 
ance, spreading over a large portion of the body and foot (PI. 5, 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 just 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. 
