MOLLUSCS. 



267 



is a shore-inhabiting species, varying in color from nearly clear yellow to dark bronze 

 green, and ornamented with a series .of radiating ridges. It likes especially shores 

 where a slight admixture of fresh J 

 water renders the sea brackish. 

 Another species. Modiolus modi- 

 olus, is lar'ger, and lives at ex- 

 treme low water mark, and be- 

 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 our shores as well as those of Europe. 



In Lithodomus the shell is small, long, and nearly cylindrical, resembling somewhat a 

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

 not by the adult, which excavates a hole in some soft rock, in which it subsequently lives. 



Fig. 278. — Modiolus pUcaiulus. 



Fig 279 — Litliodomus htliophagus in its buiiows 



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

 will be mentioned again when treating of the family Pholadidsa on a subsequent page. 

 Three species of Crenella (small thin shells with one tooth in each hinge, and stra^ht 

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



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

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

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

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

 resembling silk. 



The last number of this family which needs mention is the form which science has 

 at last decided shall be called Dreissena polymorpha. Its specific name is indicative 



