134 BULLETIN OF THE BUREAU OP FISHERIES. 



A Striking form of shell associated with the presence of parasites is that with abbre- 

 viated gaping anterior margins, the edges being much thickened and in appearance 

 rolled outward. The explanation appears to be simply that the parasites check the 

 peripheral growth of the forward portion of the mantle, or perhaps, as the result of irri- 

 tation, keep the mantle more or less retracted in this portion. The shell being controlled 

 in growth by that of the mantle, its forward extension is checked, while growth in thick- 

 ness continues. Meantime the valves of the shell, growing normally in other directions, 

 are gradually and naturally pushed apart as successive layers are added in the posterior 

 portions. In consequence, after a time the valves of the shell cease to meet anteriorly 

 when the posterior margins are apposed. The result is a shell of normal dimensions 

 behind and below but abbreviated in front, where the edges are disproportionately thick 

 and gaping. 



A very familiar form of abnormality is shown by the shells in Plate XII. When a 

 single shell of this type is first seen one is inclined to suppose that the deformity is the 

 result of a mechanical injury; but when shells marked by almost identically the same 

 abnormality are repeatedly found in various places and in different kinds of bottom, it 

 becomes evident that the explanation of mechanical injury is not applicable. It is prob- 

 able that a parasite checked the growth of the mantle at a particular point, so that, while 

 growth of shell continued normally both before and behind, it was so retarded at that 

 point that a permanently notched outline resulted. The subject of discolored and mal- 

 formed shells is not introduced, however, with the object of definitely explaining them, 

 but rather with a view to directing attention to the desirability of further investigations of 

 the parasites of mussels, as well as of certain features of the environment of mussels, as 

 regards their effects upon the form and quality of shells. 



