fact that the margins of the valves "are thicker than would other- 

 wise be expected. 



If a prismatic layer is present it must be secreted by the 

 extreme margin of the mantle but there is nothing in the appear- 

 ance of the epethelium of this region that would indicate that it 

 has a different function than that a little further removed from 

 the margin. 



A gland for the formation of the cuticle occupies a grove 

 along the margin of the mantle, (fig. 26, cgl.) and fragments of 

 a thin and apparently not very tough cuticle may nearly always 

 be seen in it attached at the inner end of the gland to the epethe- 

 lium that secretes it, but the withdrawal of the mantle margins 

 far into the shell when the shell is closed must interfere with 

 the formation of anything like a continuous cuticle. The liga- 

 ment and cartilage are both present, although the former is 

 quite thin, and may be looked upon as modifications of the 

 cuticle. They are secreted by adjacent portions of the mantle. 



The free margin of each lobe is very muscular, is abundantly 

 supplied with organs in the form of tactile tentacles and eyes, 

 and is provided with a large flattened ridge that is turned away 

 from the shell valve on which the mantle lobe rests (figs. 10, 19, 

 20, and 26) . The ridges on the two mantle lobes may be brought 

 into contact with each other at any or all points along the 

 margin, when the shell valves are slightly separated and thus 

 regulate the currents of water formed by the cilia on the gills, 



or m swimmmg. 



The muscles of the mantle, (figs. 16 and 26) may be grouped 

 as the radial pallia!, which are attached to the shell at the pallial 

 lines and radiate out toward the margins, the circular pallial 

 which are very strong, attached to the shell valves near the hinge 

 line and run along the borders of the mantle, the muscles of 

 the pallial ridge that are largely circular but contain also radial 

 muscles connected with the radial muscles that have been 

 referred to, and the suspensory muscles of the gills which are 

 really continuation of a fold of the mantle and are attached to 

 the shell valves between the pallial scars and the adductor muscle 

 scars. All but the last group of these muscles, which will be 

 described in connection with the gills, are confined to the borders 

 of the mantle and, together with the infolded ridge, sense organs 



13 



