. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 10 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY of mm., drawn with the aid of a camera lucida. Of interest is the relative thickness of the valves, the amount of overlap of the upper valve at the ventral margin, the position of the upper and lower muscle scars, and the arrangement of the hinge and ligament. Of particular interest is the method of development of the layer produced by the muscle epithelium. The adductor muscle must necessarily change position as the shell enlarges in order to maintain a somewhat centra
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 10 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY of mm., drawn with the aid of a camera lucida. Of interest is the relative thickness of the valves, the amount of overlap of the upper valve at the ventral margin, the position of the upper and lower muscle scars, and the arrangement of the hinge and ligament. Of particular interest is the method of development of the layer produced by the muscle epithelium. The adductor muscle must necessarily change position as the shell enlarges in order to maintain a somewhat central location. This is accom- plished by a ventral migration of the muscle. At the same time deposition of shell material continues in the area contiguent to the muscle so that in time a distinct middle layer dorsal to the muscle is produced. A well-defined prismatic layer is easily seen in the lower valve of the young dissoconch when viewed from above (Figs. 1 and 3; PI. 2, fig. 2). The prisms are irregularly shaped and are secreted. 0. I mm -H Figure 3. Line drawing to show prismatic structure in the right valve. in an erratic, though concentric pattern. On the average, when measured near and parallel to the ventral margin, there are about 5 prisms per mm. The prismatic pattern persists until the shell reaches about 5 to 7 mm. in height at which size the shell becomes too thick for this structure to be seen clearly. The External Sculpture. The surface sculpture differs in the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of Comparative Zoology. Cambridge, Mass. : The Museum
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