6o2 SAMPSON. [Vol. XL 



The anterior group of muscles is attached to the shell in the 

 region where it emerges from under the shell next anterior to 

 it (Fig. 3, aga ; cf. also Figs. 2, 6) ; the posterior group to the 

 region anterior to the underlying edge of the apophysis of the 

 next posterior shell (Fig. 3, pga ; Figs. 2, 6). Nephridial 

 branches occupy, as a rule, the spaces immediately ventral to 

 the shell and alternate with the groups of muscular attach- 

 ments. 



In each group, at the shell, are three muscles that corre- 

 spond in the two groups, although the exact relations of the 

 muscles to one another are not identical. The muscle nearest 

 the median line at the shell will be called (because of its 

 distribution) the "latero-pedal," the most lateral muscle, the 

 "medio-pedal," and the muscle between, the "antero-oblique." 



The inner fibres of the latero-pedal (Figs. 4, lp\, 5, Ipi) are 

 attached to the shell over the body cavity (so that they form 

 the roof of the lateral part of the cavity), curve out, and then 

 pass ventrally into the foot ; the remaining fibres of the muscle 

 are, on the whole, dorso-ventral. After leaving the shell, some 

 of the fibres diverge anteriorly and posteriorly (Figs. 2, 6, lp\, 

 Ipz) until those of consecutive anterior and posterior groups 

 meet ; the muscle is distributed to that part of the foot which 

 lies outside the pedal nerve, including the portion spread out 

 under the mantle chamber (Figs. 4, 5). The posterior fibres 

 of the latero-pedal of the anterior group, and the most lateral 

 fibres of the oblique dorsal muscle, overlap near the shell. 

 (Fig. 4 is taken from a section anterior to this region ; but cf. 

 Fig. 3, od and aga.) 



The medio-pedal muscles (Figs. 4, mpi, 5, mpz) are attached 

 to the shell over the mantle chamber, and pass obliquely inward 

 until they cross the path of the latero-pedal fibres (Fig. 6, mpi, 

 mp2)y and continue the boundary of the body cavity ; some 

 fibres are distributed to the part of the foot inside the pedal 

 nerve, and others cross the median line, and end in the oppo- 

 site half of the foot. The fibres of the medio-pedal of the 

 anterior group do not diverge anteriorly and posteriorly from 

 their attachments to the shell like those of the latero-pedal 

 muscles, but the muscle has a broader attachment antero- 



