112 



PALAEONTOLOGY OF THE UPPER MISSOURI. 



bases. Mantle simple in front; branchial plume pectinated, partially 

 exposed on the right side when the animal walks ; the laminae pinnate, 

 spirally twisted, protected by a respiratory lobe. Foot bifid anteriorly. 

 Lingual teeth (3. 1. 3) hooked and denticulate, the central series broad, 

 lateral lanceolate. 



This family embraces but the typical genus Valvata, and Lyogyrm. The species 

 are rather widely distributed in temperate regions, and inhabit lakes, ponds, ditches, 

 and sluggish streams. They are the only known Prosobranchiate Gasteropods 

 having exposed gills. 



Genus VALVATA, MTJLLER. 



Synon. Valvata, MCLLEB, Hist. Verm. II, 1774, 198 ; and Zool. Dan. Prodr. 1776, 239 ; STBDER, COXE Trav. Ill, 391 ; 

 ABILDGAABD, Skrivt. af Naturh. SELSK. 1794, III, 61 ; DBAPABN. Tabl. 1801, 30 ; and Hist. 1805, 26, 

 28, 41 ; LAMK. Extr. d'un Cours. 1812, and Hist. VI, 1822, II, 171. 



Valvearius, DDM. Zool. Anal. 1806, 164. 



Gyrorbis, FITZINGEB, Verz. 1833, 117. 



Planella, SCHLUT, Vz. 1838, 13. 



Valvata, BEBOE, Conch. Bnch, 1847, 17, 20, 26. 



Tropidina, H. & A. ADAMS, Genera Recent Mol. 1856, 344. 

 Etym. Valvatus, having folds or valves. 

 Type. Valvata cristata, MCLLEB. 



SheU umbilicate; spire usually much depressed, sometimes moderately prominent; 

 whorls rounded or carinate; epidermis corneous; aperture circular; lip thin and 

 sharp. 



The shells of this genus are distinguished from those of the allied type Lyogyrus 

 by never having the last turn free at the aperture. The group embraces two rather 

 marked sections, as follows: 



1. Valvata (proper) = Gyrorbis, FITZINOER, = Planella, SCHLUT. 



Shell greatly depressed or planorbicular ; with a very wide umbilicus and rounded whorls. 

 Example. V. cristata, MULLEK. 



3. Tropidina, H. & A. ADAMS. 



Shell turbinate, or having the spire more or less prominent, and the volutions either carinate oj rounded. 

 Examples. V. tricarinata, SAY, and V. piscinalis, MULLEK. 



As there are some slight differences in the details of the lingual teeth in these 

 two groups, as well as in the form of the shell, it is possible they may more pro- 

 perly constitute distinct genera. 



The Messrs. H. & A. Adams admit three subgenera under this genus, in their 

 valuable work on the genera of Recent Mollusca. Regarding such forms as V. 

 piscinalis as typical, they adopt Fitzinger's name Gyrorbis, for such types as V. 

 cristata, Miiller, and propose the name Tropidina for forms like V. tricarinata, Say. 

 As the genus Valvata, however, was originaUy founded by Miiller, upon V. cristata, 

 that species must be regarded as the type, and as Gyrorbis was also founded upon 

 the same shell it must be viewed as exactly synonymous with Valvata proper, and 

 cannot be used for another group. Although Tropidina was proposed for the recep- 

 tion of carinated forms only, there is no reason why it should not also include the 

 other turbinate species, with rounded whorls, and a more or less prominent spire, 

 since the carinated character is not constant even in the species tricarinata. 



