MOLLl,'SCS. 



267 



is a shore-iiiliahitini;' sjjecics, varying in color from nearly clear yellow to dark bronze 

 green, and ornamented with a series of ra<liating ridges. Il likes es]ieeially shores 

 whore a slight admixture of fresh 

 water renders the sesi brackish. 

 Another species, Modiolus modi- 

 olus, is largei', and lives at ex- 

 treme low water mark, and lie- 

 low. The surface of the shell 

 is not ribbed, but specimens 

 from sheltered localities have 

 the epidermis of the external 

 surface produced into bristles 

 and hairs. It occurs on ovir shores as well as those of Euro|ie. 



In LithodoiHus the shell is small, long, and nearly cylindrical, resendjling somewhat a 

 date ; and is covered with a thick, dark epidermis. In the young a byssus is spun, but 

 not Ijy the adult, which e.xcavates a hole in some soft rock, in which it subsequently lives. 



Fig. '!-». — Moilinhis pi 



P'iG. 279. — Lithodomus fithophaf/us in its burrows. 



Like all rock-excavating forms, it is not kmiwn linw it bores its holes, a question which 

 will be mentioned again when treating of the family I'hola<lida^ on a subsequent page. 

 Three species of Crevellu (small thin shells with one tooth iu each hinge, and straight 

 beaks) are found in our northern shores, one extending to the south of Cape Cod. 



The species of Pinva have long, triangular shells, tapering to an acute angle at the 

 umbones. The shells are very thin and delicate, and are usually oriuimented exteriorly 

 by large or small scale-like projections. The animals spin a very large and strong 

 byssus, which, as a curiosity, lias been used in textile arts, the product somewhat 

 resembling silk. 



The last luunber of this family which needs mention is the furm which science has 

 at last decided shall be called Dreisxciia ])ol>/morj)/i<i. Its sjiecitic name is indicative 



