402 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Recorded also from Hell-Gate River, Montana (Cooper) ; Utah; Ma- 

 lade River, Idaho (Hemphill); Washington Territory ; Yellowstone 

 River (Smithsonian Catalogue); British possessions (Lord) northward. 

 Cosmopolitan. 



PLANORBINiE. 



Helisoma plexata, sp. no v. 



Shell a little, larger than P. Trivolvis, Say, of irregular proportions, 

 fragile; whorls 4-5, the inner 3-4 of the spire' angulated and coiled in a 

 plane, which is considerably inclined to the plane of the outer revolution in 

 such a way that the carina of the third whorl rises into a sharp shoulder on 

 the right side, and on the left side sinks underneath the overflowing last whorl, 

 which takes on a sudden increase in old age. A similar, but less, change in 

 the plane often occurs again in the fourth whorl, giving a very twisted ap- 

 pearance to the shell. Surface marked by irregularly -crowded, wavy, raised 

 lines of growth. Umbilicus broad, exhibiting the well-rounded whorls to 

 the apex. Aperture somewhat oblique, pretty regularly pyriform in outline, 

 the vertical slightly exceeding the horizontal diameter, and embracing a con- 

 siderable portion of the body whorl, well to one side of the median line. 

 Peristome gently reflected, slightly thickened within, and fully lined with 

 an opaque ivhite deposit, which also forms a thick and well-defined callous 

 connecting the ends. Color yellowish horn to reddish-brown (becoming 

 almost black behind the aperture), most specimens abundantly banded and 

 streaked ivith revolving lines of ochraceous red, and fine black threads. 



Spire. Aperture. Section at aperture 



showing change 

 of plane in revolu- 

 tion. 



Saint Mary's Lake, Antelope Park 25 specimens. 



This species existed in countless numbers in the above-mentioned 

 lake, which is a small sheet of water held among precipitous cliffs, 

 that afford it no visible outlet. It seems to be merely a " sink " for 

 the melted snow of the surrounding heights. All of the hundreds of 

 individuals seen, possessed, in a more or less marked degree, the 

 twisted appearance, resulting from the change of plane in the old age 

 of the shell, which is their most striking character. How the species 

 came, almost entirely alone, to inhabit this secluded lake is a problem, 

 complicated by the fact that there probably is not another large Plan- 

 orbis within fifty miles. That the wild fowl, abundant on the lake, 

 brought the eggs, clinging to their feet, may be a plausible explanation; 

 but where did they bring them from, and when? The bottom of the 

 lake is, for the most part, rough conglomerate rock, and it is in many 

 places filled with heavy water-plants, which may account for the pecu- 

 liarities of the shell. 



The members of the family Planorbidw, seem to be particularly sub- 

 ject to sudden and eccentric deviations from the normal form of the 

 group. Many curious examples have been noticed. The genus Valvata 

 seems also subject to similar deformities, which Prof. Alpheus Hyatt, of 

 Cambridge, Mass., has been paying special attention to of late, in the 



