59S SAMPSON. [Vol. XL 



(2) A pair of oblique dorsal muscles (Figs. 2, 3, 4, 5, 6, od) 

 attached with the median dorsal muscle to the anterior edges 

 of shells VIII-II, inclusive. These muscles run obliquely 

 forward, at first along the edges of the apophyses of the shell 

 to whose median anterior edge they are attached, and ante- 

 riorly they are attached to the next anterior shell (under which 

 they have taken their course) on the line where its apophyses 

 begin (Figs. 2, 3, od)\ e.g., one of these muscles is attached to 

 the anterior edge of V, runs obliquely forward under IV, and 

 is attached to the ventral surface of IV where its apophyses 

 begin. Both the median and oblique dorsal muscles are want- 

 ing under VIII, as no shell posterior to it occurs. Under II, 

 the median dorsal muscle is comparatively narrow and thick 

 (Fig. 10, Did), for it is crowded toward the median line by a 

 group of muscles (dr) extending from II to the radula ; under 

 the more anterior part of the shell, the fibres diminish in 

 number, so that the anterior limit of the muscle is at the 

 beginning of the curvature of the shell over the end of the 

 body {cf. Fig. 3 and description of the shells). The anterior 

 attachment to the shell of each of the oblique dorsal muscles 

 under II is not continuous, but the fibres are attached in 

 groups that alternate with the attachments of groups of fibres 

 of pedal muscles (v. infra, see Fig. 3) : this is also true to some 

 extent of the corresponding muscles under the other shells. 

 Under shell I are two pairs of muscles that pass anteriorly 

 (and also ventrally, following the shape of the shell) from the 

 anterior edge of II ; the muscles of the median pair diverge 

 but slightly from their attachment to II, and represent the 

 median dorsal muscle (Fig. 3, md)\ the more lateral pair (Fig. 

 3, od) corresponds to the oblique dorsal muscles of the other 

 shells, but each muscle of the pair is divided very near to its 

 attachment to II into two main strands ; the outer fibres of 

 the outside strands end at the shell with alternating groups of 

 fibres of a dorso-ventral muscle {cf. Fig. 3). 



(3) At the lateral edges of the shells, longitudinal muscles 

 (Figs. 2, 3, //, and also // of cross-sections) on either side 

 which pass from the ventral surface of one shell to the dorsal 

 surface of the apophysis of the next posterior shell. As in the 



