have been unable to determine that they have a special function 

 or that they are specially advantageously placed. 



The outer surfaces of the valves (figs. I to 4), are marked 

 by fine ridges and grooves that radiate from the beaks to the 

 margin, and by rather prominent lines of growth that run out 

 along the hinge line. Not all of the radial markings start from 

 the beak but new ones are added at intervals so that the number 

 remains approximately from 30 to 35 per inch on any portion of 

 the surface. The lines generally have a somewhat wavy or zig- 

 zag course. Those on the upper, left valve, are more rounded 

 and prominent than those on the under, probably because of the 

 difference in wear in the two cases. The wear is further indi- 

 cated by the fact that the markings may be nearly or quite 

 obliterated on the most convex portion of the lower valve. 



The lines of growth are visible as very fine lines all over the 

 surface, but are much more prominent in some places than 

 others. The larger ones may appear as ridges, which would 

 seem to indicate that marginal growth occasionally stops as the 

 shell thickens along a line, or they may appear as a series of 

 rather jagged depressions that indicate where the old margins 

 have been broken. These latter markings are rather more 

 abundant on the lower than on the upper valve. Their relative 

 abundance compared to the shells of other lamellibranchs is 

 easily accounted for by the use to which the shell is put in swim- 

 ming and as the upper valve usually extends over the margin 

 of the lower (fig. 19) each blow when the valves are clapped 

 together would be directed by the more solid inner portion of 

 the upper valve directly onto the margin of the lower valve, 

 thus being more likely to break off small bits from it. 



The outer surface of the shell, especially of the upper valve, 

 seems to be more than ordinarily subject to the attacks of the 

 boring sponge, Cliona sulphurea, which frequently riddles the 

 shell so that hardly a trace of the original surface is left (fig. 3). 

 The reasons for the attacks on this shell more than on the shells 

 of other mollusks found in the region is probably due to the fact 

 that this animal does not burrow and that the shell is not pro- 

 vided with anything like an adequate cuticle, a layer that could 

 not be formed because the enlarged margin of the mantle must 

 necessarily be withdrawn far into the shell whenever the valves 



8 



