ANOMIA. 



plates, which are alternately high and low; and the stri- 

 ated appearance of the top or outer covering is pro- 

 duced by the edges of the higher plates. This appendage 

 is capable of receiving a high degree of polish, and in 

 that state it resembles ivory and is equally close-grained. 

 In the fry the orifice is larger in proportion to that of 

 the adult, and is placed on one side. The beak of 

 young specimens is sometimes much produced, and at 

 other times slightly incurved. When the shell is thin, 

 the long muscular scar seen through the upper valve 

 resembles a white line. The varied and nacreous hues 

 of the shell rival in lustre those of an opal. A group 

 of these specimens from Lulworth Cove, on a valve of 

 Pecten opercularis, now before me, are of diflerent 

 colours, white, yellow, and pink, and reflect their pearly 

 gleams in every direction. In substance the shell bears 

 some affinity to talc. Specimens from Baiitry Bay, 

 Lough Strangford, and Exmouth roads are larger than 

 usual. One from the first-named locality measures four 

 inches in diameter. Now and then, but rarely, the 

 upper valve is flat, and the lower or perforated valve is 

 convex ; and in one case the front half of the shell is 

 divided into two distinct lobes, owing to the continual 

 obstruction and irritation caused by a small branch of 

 Sertularia abietina, which had insinuated itself and 

 grown up in front of the Anomia. But a more curious 

 instance of an adaptation to circumstances is presented 

 by specimens which I found many years ago on a mus- 

 sel-bed in Swansea Bay, laid bare by an unusually low 

 tide. The orifice in every specimen was completely 

 closed by a series of thin vaulted plates of the same 

 material as the shell. All the specimens were living, 

 and attached to the mussels by the byssal threads of the 

 latter. It appeared to me that, having been acciden- 



c 5 



