SYKES : VARIATION IN KECENT MOLLUSC A. 



267 



distinct rows of these perforations. He conjectured, as I understand 

 it, that the edge of the mantle was notched either from birth or by 

 accident ; it seems to me that it may belong to a different category 

 and may be due to duplication of the series of ' tentacles.' 



The number of perforations is variable. In a series of Haliotis 

 tuherculata sent from the Channel Islands, consisting of 97 specimens, 

 all of ' merchantable' size, collected in February, I find the following 

 variation : — 



The result is of interest, though the series examined was small. 



Pelecypods are liable to various deformations. Sometimes one valve 

 is flattened, the other being inflated ; again, a sinuation in the margin 

 will occur, marked by a groove running towards the beaks, and this 

 notch has been occasionally noticed to be present in one valve only. 

 A curious specimen of Modiolaria discors that I have, has apparently 

 had one end broken off, and has formed a new shell inside the old 

 broken portion, lacking the characteristic sculpture, and with the 

 edges not properly opposed. Pecten has been seen with a notched 

 shell edge, this being due to the growth of SerpuliB. An abnormal 

 outgrowth has been seen on the edge in Mactra,^ and various deforma- 

 tions of Anodonta have been recorded, notably one in which the edges 

 of the valves on the lower margin are folded inwaiTls.^ 



As to the sculpture, Cardium is sometimes found with the margin 

 duplicated, the effect being as if one shell were placed inside another ; 

 it has also been seen with the ribs dislocated, probably by a former 

 damage.' The ribs in Pecten may divide late in life, or may arise 

 then independently, and ridges may appear inside with no corre- 

 sponding rib on the outside. 



The internal colouring of the shell is liable to vary greatly, and 

 Anodonta has been seen with the nacre of one valve white and the 

 other of a salmon colour. 



The hinge has been observed to be abnormal, and a case has been 



1 Leidy: Proc. Ac. Philad., 1879, p. 198. 



^ Sci. Gossip, vol. xxiv, p. 127. 



3 Martens: S.B. Ges. Nat. Berlin, 1879, p. 74. 



