402 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Eecorded also from Hell-Gate Eiver, Montana (Cooper) ; Utah; Ma- 

 lacle Eiver, Idaho {EempMll)', Washington Territory ; Yellowstone 

 Elver {tSmithsonian Catalogue)', British possessions {Lord) northward. 

 Cosmopolitan. 



PLANORBIN^. 

 Helisoma plexata, SP. NOV. 



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

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

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

 such a wail that the carina of the third ivhorl rises into a sharp shoulder on 

 the right side, and on the left side sinlcs underneath the overfloinng last whorl, 

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

 the plane often occurs again iji the fourth ivhorl, giving a very ticisted ap- 

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

 lines of groicth. Umbilicus broad, exhibiting the tvell-rounded ichorls to 

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

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

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

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

 an opaque trhite deposit, ichich also forms a thick and well-defined, callous 

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

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

 streaked with 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 thei^e 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 



