Manchester Memoirs, Vol. lix. (191 5), No. 4. i/ 



4. Marginal Notching of the Valves. 



In collecting the present shells it was found that 

 some individuals showed curious notches or incisions on 

 the margins of the valves. About fifty individuals were 

 found possessing this peculiarity, this working out at five 

 per cent, of the shells obtained. Shells in all stages of 

 growth are represented among the fifty individuals, but 

 in no case does the notch, or the evidence of a notch — as 

 shown by a narrow sinus on the shell — extend quite up 

 to the umbo — i.e., the notch was not present in the very 

 young shell. Certain shells have only one notch, whilst 

 others possess two. When one notch only is present, 

 there is considerable variation in its position. Much the 

 commonest type is that in which the notch is to the left- 

 hand side of the median line (viewed dorsally). In a 

 very few cases the notch is on the anterior margin of the 

 valves, exactly opposite the umbo ; in a few cases, again, 

 the notch is to the right of the median line, and in three 

 shells there are two notches. 



The origin of these notches is obscure, but they are 

 probably due to some injury to the mantles of the indi- 

 viduals in which they are developed, an injury possibly 

 caused by some parasitic organism. 



A similar feature is to be seen occasionally in several 

 living species of Brachiopods, e.g., Magellania venosa and 

 Magellania fragilis, and in these cases is probably due to 

 a similar cause, hence by examination of such living 

 types it might be possible to determine the factor pro- 

 ducing this curious deformation. 



