ON MARGINAL TROCESSES IN SUELLS. 1057 



46. Tlie Maro-inal Processes of Lamellihnmcli Shells. 

 By Cyiul Crosslani:), F.Z.S. 



[Received May 27, 1911 : Read June 27, 1911.] 

 (Plate LX.* and Text-figs. 229 & 230.) 



The existence of the ornamental projections on the surface of 

 so many lamellibranch shells is rather puzzling to the student 

 of Bionomics. Especially to one interested in the rate of growth 

 of mother-of-pearl shell does their formation seem a sad waste of 

 shell-ma,king energy. 



The drawings given show examples of these processes in young 

 and adult shells. Inspection reveals one striking fact, that the pi'o- 

 portion of shell-liuilding energy thus expended is very much greater 

 in quite young shells than in the mature ones of several species. 

 Compare, for example, the figure of a specimen of Alargaritifera 



Text-fie-. 229. 



SliL'lls of Murex ramosits and upper valve of adult Chama fuUata. 



Half natural size. 



Tlie illustration will serve for identification of these common shells by 



non-conchological readers. 



ifiargaritifera (the large mother-of-pearl oyster) seven months old 

 and that of the portion of the margin of one 5g years old, also the 

 young of Chama foliata and Chama sp. of about the same age 

 with the adult shells, and it is seen that relatively the processes 

 are gigantic in young shells (PI. LX. and text-fig. 229). They 

 must have their vise, or they coidd not be so greatly developed at 

 this, the second, critical stage of the bivalve's existence t (the fii'st 



* For explanation of the Plate see p. 1061. 



t Utility is suggested also by the fact that such processes cannot be regarded as 

 ancestral features such as in some other cases are more prominent in the young of a 

 species. 



