582 



W. J. Ceozier, 



the shells do not all grow at the same rate, or that the dark bands 

 do not represent winter periods, or both; Isely's (1913) results on 

 the growth of fresh-water mussels indicate that both possibilities 

 are probably realized. 



Some interesting- shell abnormalities were found. Apparently, 

 when a shell has been injured at the growing edge of the mantle, 

 the continued deposition of new material follows any unevenness 

 at the edge of the wound, so that shells resembling- that shown in 

 Plate 35 Fig. 2 are formed. Fig. 2 I'epresents an extreme case among 

 the specimens examined, but several other examples of the same 



Depth 



Interadductor 

 distance 



Palliai sinus 

 depth 



Width 



3 4 5 6 7 



Fig. D. Graphic representation of results. 



Length cms 



