MUSCULAR SCARS. 
00 
the shell. The cicatrix of the anterior pedal retractor is contiguous 
to and joined with tire anterior adductor impression, at its upper 
mai’gin. The impression of the anterior pedal protractor is much 
smaller than that of the anterior adductor scar, and is placed pos- 
teriorly and veutrally to it. The posterior adductor impression is 
joined at its upper angle to that of the posterior pedal retractor. 
In addition to these, the more distinct marks of muscular attachment, 
there are a limited but variable number, of much smaller muscular 
pits in the umbonal region, about e<pially distributed in size and 
number upon the anterior and posterior sides of the umbones, and 
not always exactly corre.sponding either in number or position in the 
two valves, marking the points of attachment of several retractor or 
retentor muscular fibres. 
■UT 
a. ad. anterior adductor scar; p.ad. posterior adductor scar; a.r. anterior pedal retractor scar; 
posterior pedal retractor scar; ii.r, umbonal or lesser retractor or retentor scars; a.p. 
anterior pedal protractor scar, the abdominal retentor scar of Clessin ; p.l. pallial line. 
Moquin-Tandon asserts that the anterior adductor impressions are 
always less-marked than those of the pofsterior adductors, this is 
however incorrect, as the anterior adductor impression, especially in 
JJnio, is usually much more deeply and distinctly impressed than 
the posterior one, which, though somewhat larger in extent is not 
nearly, so distinctly and strongly defined. 
All these marks of muscular attachment become further apart and 
more distant from the umbonal region as the growth of the shell pro- 
ceeds, and in some cases it is possible to trace faint and gradually 
