6o4 SAMPSON. [Vol. XI. 



relative anterior and posterior limits of their attachments ; in 

 the anterior group a fourth muscle occurs not represented in 

 the posterior group. Fibres in the thick part of the foot that 

 in cross-section appear to be the cut ends of longitudinal 

 muscles, are in reality the continued fibres of the oblique 

 muscles, and arise in part, also, from a horizontal muscle 

 from VIII and an oblique muscle from I and II that extend 

 a long distance into the foot. Both will be described among 

 the muscles of the shells to which they belong. Longitudinal 

 fibres which are independent of all these muscles occur in the 

 most ventral part of the foot and in its lateral edge ; and, in 

 the network of muscles in the ventral part of the foot, are 

 additional diagonal and horizontal fibres in the transverse plane 

 (see cross-sections). This general description applies to the 

 three shells, IV, V, and VI. 



Shell III. 



The arrangement under shell III is scarcely modified. The 

 antero-oblique muscles, in this more anterior part of the body, 

 are reduced in extent, and the fibres are perhaps more perpen- 

 dicular in direction ; on this account the antero-oblique fibres 

 of the posterior group are in cross-section less distinguishable 

 than in the typical case from those of the latero- and medio- 

 pedal muscles. The postero-oblique muscle, on the other hand, 

 is large and conspicuous ; the corresponding muscle of II is 

 also large, and still prominent at its lower level under III. 



Shell VII. 



Under VII the genital duct crosses to its external opening 

 into the mantle chamber beyond the gill, anterior to the attach- 

 ment of the medio-pedal of the anterior group, but ventral to 

 the anterior part of the attachment of the postero-oblique. 

 Thus the attachment of the medio-pedal is interrupted, and 

 the medio-pedal does not extend anteriorly beyond the latero- 

 pedal (Fig. 13, VII). Posterior to the anterior group of 

 muscles the auricle extends to the branchial vein and the 

 nephridial duct to its external opening into the mantle cham- 



