70 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



covered by an epidermis often bearing/filaments; beaks depressed and placed 

 near the anterior extremity, but not properly terminal; ligament linear, occu- 

 pying a marginal groove; hinge generally edentulous, sometimes with a few 

 nearly obsolete teeth, or with obscure crenulations in front; muscular and 

 pallial impressions faintly marked, the scar of the anterior muscle being 

 much smaller than that of the posterior. 



In this genus there are two more or less strongly-marked subgenera, 

 that may be defined as follows: 



1. volsella, Scopoli (typical). 



Shell smooth or concentrically striated ; hinge edentulous, or 

 with only obscure teeth. — (Type as already stated.) 



2. BRACHYDONTES, SwahlSOn. 



Shell with surface radiately or divaricately costate or striate; 

 hinge-margin more or less angulated, with the anterior slope crenu- 

 lated. — Mytilus sulcatus, Lam. 



H. and A. Adams propose another subgenus, Adula, characterized by 

 nearly cylindrical shell, with subcentral beaks, the type being M. soleniformis, 

 d'Orbigny; but it is probable that this can better be eliminated as a distinct 

 genus. 



The genus Volsella, or Modiolus, differs from Mytilus, Linn., in having 

 the beaks placed more or less back from the anterior end and obtuse, instead 

 of being pointed and quite terminal. The anterior basal margin of these 

 shells is also always more prominent than in Mytilus ; while Volsella is further 

 distinguished by its habit of forming a kind of woven nest, or burrow, com- 

 posed of small fragments of stones and shells. 



Species having the external appearance of this genus occur in the 

 Paheozoic rocks, though little is known in regard to their hinge and interior. 

 Some of those from the Devonian, and more especially those from the Carbo- 

 niferous, so closely resemble typical species of the genus as to leave little room 

 for doubts in regard to the propriety of placing them in it. It also occurs in 

 the Triassic rocks, ranges through all of the later formations, and is abun- 

 dantly represented in the existing seas, where it probably attains its greatest 

 development. The recent species are chiefly found in southern latitudes, 

 though a few occur on the coast of Great Britain and in the Arctic seas; also 

 on the coast of the United Slates, &c. 



