INVERTEBRATE PALAEONTOLOGY. f)89 



Limnaa in not having the columella twisted or provided with a fold, as well 

 as in solidity and texture, and the form of the aperture. 'In size and general 

 appearance, it agrees rather nearly with Bithinia ; but its aperture differs in 

 being produced, very narrow, and slightly sinuous in front, instead of regu- 

 larly rounded, as well as in wanting an opercular ridge just within the edge 

 of the lip. It is also a rather thicker shell than the typical Bithinias, and 

 has the margin of its outer lip more prominent below the middle. 



From all its characters yet known, it would seem most probably to 

 belong to the genus Campeloma. The only objections to this conclusion are 

 the comparatively small size and slender form of the shell, which, however, 

 may probably only be strongly-marked specific characters. .Still, even in 

 size, the larger specimens agree better with Campeloma than with Bithinia.^ 



The revolving strise mentioned in the description are so very minute 

 and obscure that they can only be seen by very careful looking with a good 

 magnifier. 



Locality and position. — Fort Clarke, Dakota, on the Missouri River; 



and near the mouth of Yellowstone River, Montana, in the Fort Union 



Lignite group. 



VALVATID^E. 



Genus VALVATA, Muller. 



Synon.— Valvata, Muller (1774), Hist. Verm., II, 196; and (1776) Zool. Dan. Prodr., 239.— Studer (1789), 

 CoxeTrav., Ill, 391.— Draparn. (1801), Tab]., 30,42; and (1805) Hist., 26,28 and 41.— 

 Gray (1821), Lond. Med. Rep., 231; and (1840) Turton's Man. (2d ed.), 97.— Lamarck 

 (1822), Hist., VI, 2, 171, and (2d ed.) VII, 505.— Peshayes (1832), Encyc. M<Jtu., Ill, 

 1106.— Blainr. (1824), Diet. Sci. Nat., XXXII, 229; and Malac. (1825) 434; also 

 (1828) Diet. Sci. Nat., LVI, 462.— Swainson (1840), Malac., 169, and 339.— Menko 

 (1845), Monogr. in Zeitschr. f. Malak., 115. — H. and A. Adams (1854), Genera Recent 

 Moll., I, 343.— Binney (1865), Land and Fresh-water Shells of N. Am., part III, 8.— 

 Meek (1865), Palseont. Upper Mo., 112. 



Falvearius, Dum. (1806), Zo61. Anal., 164. 



Gyrorbis, Fitzing. (1833), Verz., 117. 



Planella, Schliit. (1838), Vz., 13 (subgenus Valvata). 



Tropidina, H. and A. Adams (1856), Genera Recent Moll., I, 344 (as a subgenus under Valvata). — 



Meek (1865), Palseont. Upper Missouri, 112 (as a subgenus under Valvata). 



• 



Etym. — Valva, the wing, or fold of a door. 

 Type. — Valvata cristata, Muller. 



Shell small, discoid or more or less turbinate, umbilicate, thin ; volu- 

 tions rounded, simple or keeled, and provided with a corneous epidermis ; 

 aperture circular, with a continuous peristome. 



H. and A. Adams propose to divide this genus into three subgenera — 

 that is, into Valvata proper, Gyrorbis, and Tropidina. As Gyrorbis, how- 



