LYMN^IME OF NORTH AMERICA. 247 



Pupilla muscorum, Succinca grosvenori, Vallonia gracilicosta and Pla- 

 norbis parvus. Numerous exposures disclose the recent geological 

 history of the region to perfection. The South Platte first excavated 

 a rather deep valley with steep bluffs along the northwest edge, then 

 it refilled that valley with perfectly stratified deposits for the most part 

 of fine material, with some local gravel beds. It is in a stratum of 

 the fine material that these shells have been found." 



Ecology : Parva is to be searched for in wet, marshy places, 

 generally out of the water, on sticks, stones or muddy flats. The animal 

 is more prone to leave the water than any other of the Lymnasas. 



Remarks : Parva may be known by its peculiar bulimoid appear- 

 ance. It is more slender than humilis, has a longer, more turreted 

 spire, a more regularly elliptical aperture and a differently shaped inner 

 lip, which is roundly and broadly reflected over the umbilicus. Some 

 specimens resemble in outline a miniature Campeloma as recorded by 

 Lea in his description of curta. Occasionally specimens will be found 

 in which the whorls are more or less shouldered, the umbilicus is more 

 widely open and the sutures are more deeply impressed. (PI. XXIX, 

 fig. 8.) The sculpture of the surface varies, in some examples the 

 shell being smooth while in others there are very fine revolving lines. 

 Parva also resembles umbilicata, but may be known from that species 

 by the shape of the inner lip, which stands erect in parva while it is 

 flattened out and excavated, especially at its junction with the parietal 

 wall, in umbilicata. The spire is longer and narrozver, the sutures are 

 more deeply impressed and the whorls rounder in parva. Parva is a 

 common species and will probably prove to be widely distributed. 

 When adult and perfectly formed it is one of the most graceful and 

 handsome of Lymnseas, with its rounded whorls, deep sutures and 

 elliptical aperture. It varies somewhat in corpulency, some specimens 

 being narrow while others are quite robust. The aperture varies from 

 almost round to long-ovate. 



Parva resembles truncatula somewhat, but is easily distinguished 

 by its more elliptical aperture and longer, more attenuated spire. The 

 umbilical region and columella are also different. Though resembling 

 humilis, it is quite distinct and was erroneously placed in the synonym 

 of that species by Binney. It is probable that a number of the records 

 of humilis were founded on parva, but as it is impossible to examine 

 the original specimens upon which the records were made, their au- 

 thenticity cannot be verified. 



The types of Lea's parva are all small, immature specimens. His 

 curta is large and robust, representing the adult form of parva. A 



