No. 3.] THE MUSCULA TURE OF CHITON. 603 



posteriorly than the latero-pedal of the same group, extending 

 beyond it anteriorly, and for some distance posteriorly (Fig. 

 13, III-VI, Ipi, vipi ; cf. also Fig. 2, Ipi, mpx of IV). 



The fibres of the medio-pedal of the posterior group, like the 

 latero-pedal fibres, diverge from the shell, and where these two 

 muscles and the antero-oblique occur side by side, it is difficult 

 to draw sharp lines of division between them, as may be seen 

 in cross-section (Fig. 5, Ipz, mpz). The medio-pedal muscle, 

 however, as a whole, is relatively more posterior than the 

 latero-pedal (Fig. 13, III-VI, Ipz, mpz ; also. Fig. 2, lp2, mp2 

 of IV). 



In the floor of the body cavity are occasional fibres that 

 stretch horizontally across, and disappear among the fibres of 

 the medio-pedal muscle on either side (Fig. 5). 



The antero-oblique of the anterior group (Figs. 2, 4, 6, ao^ 

 runs obliquely forward from its attachment between the latero- 

 and medio-pedal muscles, and the anterior and posterior limits 

 of its attachment to the shell are relatively a little posterior 

 to those of the latero-pedal (Fig. 13, III-VI, aoi, Ipi). The 

 attachments of the antero-oblique fibres of the posterior group 

 begin anteriorly outside of the attachment of the latero-pedal 

 muscle (Fig. 13, III-VI, aoz, Ipz), and more posteriorly are 

 mingled with the fibres of the latero- and medio-pedal muscles 

 (Fig. 5, aoz) ; the fibres run very obliquely forward, overtake 

 those of the antero-oblique muscle of the anterior group (Fig. 

 4, aoi, ad 2 ; cf. also Fig. 2, aoi, aoz of V), and pass ventral to 

 them into the foot. 



In the anterior group, ' still another muscle, the " postero- 

 oblique" (Figs. 2, 4, 6, po), is attached to the shell immediately 

 inside the attachment of the medio-pedal muscle, although it 

 does not extend posteriorly so far as the medio-pedal and its 

 attachment reaches relatively more anteriorly (Fig. 13, III- 

 VI,/^); this muscle runs obliquely backward inside the medio- 

 pedal of this group (Fig. 4, po), but outside the medio-pedal of 

 the posterior group (Fig. 5, po'). 



There are, then, two groups of muscles to the foot under 

 the typical shell, each composed of three muscles having a 

 similar distribution in the two groups, but differing in the 



