512 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Naidea, Swaiuson (1840), ib., 378 (as a subgenus under JEglia). — H. and A. Adams (1857), Genera 



Recent Moll., II, 492 (as a subgenus under Unio). 

 Xa'ia, Swainsou (1840), Malac, 284 and 379 (as a subgenus under Casialia). 

 MegadomuK, Swaiuson (1840), Malac, 27 and 378. 

 JEglia, Swaiuson (1840), Malac, 275 and 378 (as a genus). 

 Canihyria, Swaiuson (1840), ib., 276 and 378 (as a subgeuus under 2Eglia).—'K. and A. Adams 



(1857), Genera Recent Moll., II, 496 (as a subgenus under Unio). 

 Jridea, Swaiuson (1840), Malac, 283 and 379 (as a geuns). — H. and A. Adams (1857), Genera 



Recent Moll., II, 496 (as a subgenus under Unio). 

 Micromya, Agassiz (1852), Arch. f. Nat., XVIII, 47 (as a genus). 

 Tritogonia, Agassiz (1852), ib., 48 (as a genus). 

 Dysnomya. Agassiz (1852), Arch. f. Nat., XVIII, 1 and 43 (as a genus). — H. and A. Adams (1857), 



Genera Recent Moll., II, 497 (as a subgeuus under Unio). 

 Orthonymns, Agassiz (1852), Arch. f. Nat., XVIII, 48 (as a genus). 

 Cyprogenia, Agassiz (1852), ib., 47 (as a genus). 

 Xodularia, Lanceolaria, Coelatura, Glebula, and Uniomeris, Conrad (1853), Proceed. Acad. Nat. Sci. 



Philad., VI, 268 (as subgenera under Unio). 

 Rhipidodonla, Miircb (1853), Cat. Yoldi, 11,47 (as a subgenus under Unio). 

 Loxopleurns, Meek ( 1872), in Hayden's Second Annual Report TJ. S. Geological Survey of the 



Territories, 294 (as a subgeuus under Unio). 



EUjm.— Unio, a single large pearl 

 Type. — Mya pictorum, Linn. 



Shell varying in form, according to the species and subgenera, from 

 transversely-oval, elliptical, oblong, or lanceolate, to snbtrigonal, trapezoidal, 

 subcircular, &c. ; surface covered with a thin olivaceous epidermis, some- 

 times striped with greenish and olive bands, and either smooth, striated, 

 costated, plicated, tubercular, or nodular— very rarely bearing a few spines; 

 beaks often eroded ; nacre white, yellowish, flesh-colored, or various shades 

 of purple ;* hinge generally with two anterior teeth in one valve, and one in 

 the other; posterior teeth elongated and laminar, usually single in one valve 

 and double in the other. 



From the foregoing diagnosis, and t he long and tedious list of names 

 mentioned in the synonymy, it will be understood that I have given some- 

 what wide limits to this genus. This has not been done, however, so much 

 from a decided conviction that all of the proposed sections are really con- 

 generic with Unio proper, as from the great difficulty in applying most of 

 these distinctions in the classification of fossil species. Professor Agassiz, 

 some years back, proposed to separate many of the sections named by Rafi- 

 nesque and Swainson, as well as some named by himself, as distinct genera. 



* The entire substance of the shell in this genus, excepting a thin outer fibrous layer, is pear- 

 laceous. This outer fibrous layer may have been previously noticed by others; but the first published 

 notice of it that I can remember was by Dr. C. A. White, in the American Journal of Science, in 1868. 

 It is most readily seen in fossil shells, or those in a decomposing condition, in whicb it presents, under a 

 magnifier, very much the appearance of the fibrous external layer in the geuus Inoceramus. It is prob- 

 ably rather a family character than a peculiarity of the genus Unio, though it may not be so well devel- 

 oped in the other genera of the family. 



