298 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



RHIPIDOGLOSSA. 



TROCHID^E. 

 Genus MARGARITA, Leach. 



Synon. — Trochus aud Turbo (sp.) of some, l>ut not as properly defined. 



Margarita, Leach (1819), Journ. Phys., LXXXVIII, 464.— Brod. and Sowerby (1829),Zoiil. Journ., 

 IV, 363.— Gould (1841), Invert. Mass., 252; and (1870) 2d cd., 278.— Keeve (1841), Proc. 

 Zool. Soc, 76; ami (1842) Gondii. Syst., II, 108.— II. 1'. C. Moller (1842), Ind. Moll. 

 Grcenl., 8; aud (1843) Naturh. Tidssk., IV, 81.— Loveu (1846), Ind. MolLScand. Occ, 

 20.— Philippi (1847), Zeitschr. f. Malak., 20 (as a subgenus under Trochus). — H. and A. 

 Adams (1854), Genera Recent Moll., I, 433— Meek and Haydeu (1860), Proceed. Acad. 

 Nat. Sci. Philad., XII, 185.— Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 

 18.— Gabb (1864), Palaeout. Cal., II, 85— Tryon (1873), Amer. Mar. Couch., 87 (not 

 Margarita of Lea, 1838). 



EUjm. — /lapyapin/i, a pearl. 

 Examp. — il. helicina, Fab. (sp. ). 



Shell small, varying, according to the species, from subglobose to sub- 

 turbinate, or rarely to subovate, thin, nearly always umbilicate, the umbilicus 

 being without crenate margins; surface not colored; nacre of interior 

 generally brilliantly iridescent; volutions rounded, or last one rarely sub- 

 angular around the periphery; aperture circular; lip sharp, not quite con- 

 tinuous on the inner side above ; columella often ending in a simple point 

 below; surface striated, and sometimes with revolving ridges, or rarely 

 nearly or quite smooth. 



The shells of this genus seem to be nearly related to Solariella of Wood, 

 founded on S. maculata, Wood, from the English Crag. They differ, how- 

 ever, in wanting the conspicuously crenate margin around the umbilicus seen 

 in that genus, and in having a more rounded aperture. They are generally 

 quite small shells. 



This genus probably does not date back farther than the Cretaceous 

 epoch, and but few species are yet known from rocks of that age. It is more 

 abundantly represented in the Tertiaries, and attains its maximum develop- 

 ment at the present time, mainly in northern and antarctic seas. Some ten or 

 twelve recent species have been described from the northeastern coast of the 



United States. 



M argarila IV e b r a s «• eusis, M. & H. 



Plate 19, figs. 8, a, b, anil 9, a, b. 



Turbo Nebrascensis, Meek ami Haydeu ( 1856), Proceed. Acad. Nat. Sci. Philad., VIII, 64. 

 Margarita Nebrascensis, Meek and Haydeu (1860), ib., XII, 185. 



Shell turbinate, thin: length and breadth nearly equal ; spire depressed- 

 conical; volutions five, rounded or with last one sometimes slightly angular 



