greater portion of this line radiate toward the free edge of the 

 mantle. The margin of the mantle is also well supplied with 

 circular muscles that, near the hinge line, are collected into large 

 bundles and attached to the shell, (aps. and pps.). The scars 

 on the shell caused by the attachment of these muscles are some- 

 what larger and more distinct posteriorly than anteriorly. 



Immediately beneath the scars of the adductor muscle there 

 is frequently to be seen an indistinct line that marks the attach- 

 ment of the muscles by which the gills are elevated (fig. 7, sms.) 



The inner surface of the shell is sometimes quite smooth, in 

 which case fine radiating lines, possibly corresponding to the 

 radial pallial muscles, may sometimes be seen running from the 

 pallial line toward the free margins of the shell. It more fre- 

 quently happens, however,' that the inner surface of the shell 

 is roughened. These roughenings are no doubt always due to 

 the secretion of nacre caused by irritation, probably sometimes 

 due to the boring sponges before mentioned, but also to other 

 causes. Not infrequently the inner surface of the shell is 

 marked by minute elevations each of which has a dark spot at 

 its tip. In such shells it has frequently been noticed that the 

 mantle is spotted as if diseased but whether the spots are due to 

 parasites as was determined by Jameson (13) has not been 

 determined, neither has the relationship of the spots on the 

 mantle to the spots on the shell been determind. 



Pearls are sometimes very numerous, several dozen having 

 been taken from a single shell. When these become attached to 

 the inner surface of the shell, they give it a very rough appear- 

 ance. 



This inner surface of the shell is further frequently marked 

 by dark streaks and blotches that are usually caused by worms 

 and other marine animals that inhabit the holes that have been 

 formed by the boring sponges. 



~ The structure of the shell does not correspond to the structure 

 of a large majority of moUusk shells. Most shells are com- 

 posed of a nacreous inside layer, a prismatic middle layer and an 

 outside cuticle. In this form, and this holds true for the other 

 scallops that I have examined, only the nacreous layer seems to 

 be present. I have not been able to find a trace of either of the 

 other layers in the sections that I have made, but the shell is so 



10 



