G8 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



1. mytilus, Linn, (typical). 



Shell equivalve, with surface merely marked with concentric 

 striae. — (Type as already stated.) 



2. aulacomya, Morch (r= Hormomya, Morch). 



Shell equivalve, with surface radiately CQstate or striate. — M. ma- 

 gellanicus, Chem. 



3. STAVELIA, Gray. 



Shell slightly inequivalve, with ventral edge sinuous. — M. tortus, 

 Dunker. 



The genus Mytilus has been supposed to date back to palaeozoic times, but 

 it is doubtful whether it really existed before the Triassic period. Certainly 

 the so-called Carboniferous and Permian Mytili seem all to belong to the 

 genus Myalina, or possibly, in some instances, to other genera. Some species 

 of Myalina nearly simulate Volsella (— Modiola) and Mytilus, but they can 

 be readily distinguished from these genera by having a l>road, flat, longi- 

 tudinally-furrowed cartilage-area, and a distinctly fibrous outer layer like that 

 of Inoceramus. They are also nearly always inequivalve, and sometimes very 

 decidedly so. 



Species of Mytilus are found in all formations from the Trias upward, 

 and are rather numerous in our existing seas, where the genus probably 

 attains its greatest development. The existing representatives of this genus 

 have a world-wide distribution, being found from Cape Horn to Behrinir's 

 Straits, in the Black Sea, on the eastern coast of North America, the Cape of 

 Good Hope, New Zealand, &c. 



This genus presents one of the numerous instances of the great difficul- 

 ties that would be met with in applying, in Conchology, the rule followed by 

 some in other departments of natural history, of always taking the first species 

 ranged under a genus by its author as the type of the same; Linnaeus's first 

 species of Mytilus being an oyster, while his first species under Ostrea is 

 Pecten maximus! Under Mytilus, he included, along with species belonging 

 to the genus as now restricted, others belonging to Meleagrina, Lithodomus, 

 Sazicava, Modiola, Avicula, &c.; and none of those who followed him restricted 

 it to the limits now assigned it, until we come down to Lamarck (1799), and 

 he cites under it but a single example, Mytilus edulis, Linn., which therefore, 

 according to the rule most generally followed in such cases, becomes the type 



nt tile melius. 



