590 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



ever, was, according to Herrmannsen, founded on V. crhstata, Muller, it 

 becomes an exact synonym of the typical section of the genus, and cannot be 

 retained as a separate subgeneric name. Consequently, as here used, the 

 genus is understood to include only two sections, as follows: 



1. valvata, Midler (typical, = Gyrorbis, Fitz., and Planella, Schliit.). 



Shell discoid ; volutions rounded ; umbilicus very large. — (Type 

 as already cited.) 



2. tropidina, II. and A. Adams. 



Shell turbinate ; volutions rounded, with or without carinae. — 

 ( Valvata tricarinata, Say.') 



Although the section Tropidina was only proposed by H. and A. Adams 

 •for the reception of V. tricarinata, Lesueur, and V. bicar'utata, Say (which 

 latter is now generally regarded as only a variety of the first), there seems 

 to be no good reason why it may not include all of the turbinate species ; 

 because the carinate character of the volutions cannot be regarded as being 

 of much importance, since it is not always well marked, even in the V. tri- 

 carinata. 



This genus of small shells has much the habits of Limncea, being found 

 living in fresh-water ponds and ditches, and occurs both in Europe and 

 North America. According to Dr. Lea, the animal in this genus has the 

 habit of swimming inverted at the surface of the water, like Limncea, Plan- 

 orbis, and Physa. 



The geological range of the genus Valvata is not very well determined ; 

 but forms apparently nearly resembling it have been found in Jurassic rocks, 

 and it is said to occur in the Wealden of England. It is found in the Ter- 

 tiary rocks both of this country and Europe. As at the present time, the 

 species do not appear to have ever been very abundant during past geolog- 

 ical periods. 



Valvata s u 1» ii m I> i I i <- a I a , M. & H. 



Plate 43, figs. 13, a, b, c. 



Planorbis subumbilicata, Meek aud Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 120. 

 Valvata xubitmbilicaUi, Meek and Hayden (1860), ib., XII, 185. 



Shell small, subdiscoidal ; spire much depressed, or rising but slightly 

 above the last whorl ; volutions two and a half to three, rounded and increas- 

 ing rather rapidly in size: suture strongly defined ; umbilicus from one-half 



