6i2 SAMPSON. [Vol. XI. 



width of the apophysis. For a short interval under the new 

 shell (apophysis) the attachment of the fibres is interrupted, 

 and in the space, blood passes to the mantle (Fig. 3). Fig. 4 

 shows a section immediately posterior to this interruption, 

 where all the fibres have not resumed their attachment. The 

 attachment is continued posteriorly along the region of the 

 attachment of the anterior group of pedal muscles {cf. Fig. 3), 

 and posterior to this a second interruption occurs where a well- 

 defined branch of the branchial vein, and also nerves, pass to 

 the mantle : these spaces interrupt only the attachments of 

 the fibres, while ventral to the spaces there is no break in the 

 continuity of the muscle. The attachment of the interior 

 mantle muscle to the shell is continuous around the anterior 

 edge of I (Fig. 3, ima, Fig. 8, im) and likewise around the 

 posterior edge of VIII. Under II, the attachment is inter- 

 rupted by the lateral protractor of the buccal mass (Fig. 3, 

 Ipra), while the oblique radula muscle passes through the space 

 that regularly occurs for the blood and nerve supply. Under 

 I two pairs of groups of horizontal muscles pass from the 

 anterior part of the shell through the interior mantle muscle 

 to the buccal cartilages. 



The mantle is further supplied by bundles of fibres that 

 radiate from the extreme edge of the shell into the fold that 

 covers its insertion and into the interior part of the mantle 

 (Figs. 4, 5, etc), by fibres that occur in the directions parallel 

 to the lateral (or dorsal) and to the ventral surfaces of the 

 mantle, and by a network of fibres in the ventral portions of 

 the mantle. Muscle fibres occur between the branchial vein 

 and artery, and the branchio-visceral nerve, and in the lamella 

 of the gills. 



Summary. 



Under IV, V, VI, are : (i) Muscles of the shell: — a median 

 dorsal and a pair of oblique dorsal muscles, attached to the 

 anterior part of each shell and extending forward under the 

 next anterior shell to be attached to it anteriorly ; a series of 

 longitudinal muscles connecting the ventral and dorsal sur- 

 faces of consecutive shells at their sides ; a muscular cushion 



