544 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



like these present so lew characters upon which we can base specific distinc- 

 tions, that it would require better specimens than those at hand to estab- 

 lish beyond doubt more than one species. 



Locality and posit ion. — Three miles below Fort Union, in the Fort 

 Union Lignite group of the Upper Missouri ; probably Lower Eocene. 



VITRINID^E. 



Genus VITRINA, Drapernaud, 



Synon.—Viirina, Drap. (1801), Tab]., 33 aud 98; and (1805) Hist., 23, 3D, and 119?— Roissy (1805), Moll., 

 V, 391.— Cnvier (1812), Aunalesdu Mus., XIX ; and (1817) Eigne An., II, 405 (as a sub- 

 genus).— Gray (18211, London Med. Rep., XV, 231; and (1840) in Turton's Man. (2d 

 ed.), 118 ; also (1842) Synon. Moll. Brit. Mus., 90 (as a genus).— Fleming (1828), Hist. 

 Brit. An., 255 and 267— Desbayes (1830), Encyc. Metb., Ill ; and (1832) ib., 1133.— 

 Sowerby, jr. (1839), Coneb. Man., 114.— Thompson (1840), Laud and Fresh-water Moll. 

 Ireland, 5.— Gray (1840), Turton's Man. (2d ed.), 118.— H. aud A. Adams (1855), Genera 

 Receut Moll., II, 120.— Biuney (1869), Laud and Fresh-water Shells N. Am., part I, 

 26. — Tryon (1866), Mouogr. Terrest. Moll. Am. Jour. Couch., II, 243; and of numerous 

 others. 



EelkoUmax (part), Fenissac (1801), Mem. Soc. MeU, IV, 390. 



Semilimax, Fe'russac (1802), Naturf., XXIX. 



ntiimis, Montfort (1810), Conch. Syst., II, 238. 



Hyalina, Studr. (1820), System. Verzeicb., 11 (not Sebum.). 



Limacina, Hartm. (1821), Neue Alpina, I, 206 (not Cuvier and others). 



Paga/ia, Gist. (1848), Natg., 168. 



Elym. — Vitrum, glass. 



Type. — Helix pellueida, Midler. 



Shell depressed, imperforated, thin, glassy, transparent, with a short 

 spire composed of two to three volutions, which increase rapidly in size, the 

 last one being dilated; aperture wide; peristome thin, simple, aud often 

 membranaceous. 



The shells of this genus are small in comparison with the animal. In 

 the recent state, they are very thin and transparent, and the animal has much 

 the same habits as many other land-shells, being found in damp places among 

 loose earth, grass, moss, dead leaves, &c. The animal is active, and 

 said by H. and A. Adams to be capable of leaping several inches from the 

 ground* when suddenly alarmed. The existing species are numerous, and 

 widely distributed over the world. Pfeiffer enumerates in his Monograph 

 eighty-four recent species from Australia and South America alone. They 

 are very much less numerous in North America, only three species being 

 admitted in Mr. Binney's Smithsonian Monograph. 



The geological range of the genus is not well known. In the Old World, 

 only some four or five fossil species have been described, and all of these 



