CHAMA, CARDIUM AND VENUS 85 



larger than a big pin head. The inflated, trigonal 

 variety is known as var. purpurea^ H. C. Lea. 



Petricola carditoides^ Conr., the Rock-dweller, is a 

 very variable creature, and often seems to be the vic- 

 tim of circumstances. Normally the shell is oval, 

 with radiating ribs; but the creature has the habit of 

 boring into soft rock, or getting into a hole that was 

 there before, and then growing to fit the premises. 

 For this reason it happens that specimens differ much 

 in external appearance. Sometimes one is long and 

 narrow, while its neighbor is shaped like a fat bean. 

 The ligament is external, the hinge-teeth strong, 

 though sometimes they are nearly obsolete, while the 

 shell becomes thick and rough. The color is dingy 

 white, and the length is an inch and sometimes even 

 two inches. 



