SCALLOPS AND PHOLADES. 243 



move by means of alternately opening and closing the 



flattish shells with a kind of jerk. This enables them 



to dart backwards to some distance ; and, as some of 



our British species have very richly coloured shells 



(especially those obtained off the Fi g . I73 . 



south-eastern coasts), it follows 



that they have a butterfly look 



when seen in the water. Around 



the edge of the mantle, through 



the partly opened shells, may be 



seen a row of eyes, looking like 



diamonds set in a ring, except 



that the latter can give no idea 



whatever of the depth of colour Scall P 



and play of light which these eye-spots of the Pecten 



seem to contain. 



The burrowing powers of some bivalves is not 

 limited to sand or mud. We find them able to 

 excavate a protection for their fragile shells in the 

 hardest rocks ; limestones, however, being evidently 

 preferred to any other. The Pholas, Saxicava, 

 Teredos, and others, are able, nevertheless, by means 

 of the rough thick part of these shells, near the hinge, 

 to mechanically excavate the holes in which we find 

 such mollusca living. The mode in which these 

 holes in rock were formed was discovered by keep- 

 ing pholades in an aquarium, by Mr. Robertson of 

 Brighton, who carefully watched the entire process 

 and wrote a detailed account of it. These holes are 



R 2 



