1881.] NATURAL SCIENCES OF PHILADELPHIA. lOY 



the reason that some of them reach a size very much in excess of 

 the largest individuals of the same species, from points east of 

 said range. 



The carinated and planulate forms seem to be freer from the 

 bulging or variceal peculiarity than the others. 



As to the relations of the various species to each other, or their 

 interrelations,, it is quite evident that many of them have an 

 immediate common ancestry. P. trivolvis (+ P- trivolvis var. 

 fallax + P. lentus) of the American species appears to be the 

 dominant stock-form, and maj' be regarded as Americanized 

 Cornells, if a serai-political term may be used in a physico- 

 geographical sense ; its presence in the company of such forms as 

 Limneea stagnalis, L. palustris and others, of circumpolar dis- 

 tribution, indicates a geographical identity with the European 

 species. 



While the Planor.bes attain their maximum of size in that part 

 of North America west of the Rocky Mountains and north of 

 latitude 30° N., the number of supposed species, or of forms 

 which present characters more or less distinct, are more numerous 

 east of said range. 



There is apparently no relation between altitude of habitat and 

 size of shell. The quantity of West-coast material accessible at 

 this moment is too small to enable me to give a satisfactory 

 exhibit of measurements. The following will, however, convey a 

 fair idea of the robust proportions of the more conspicuous West- 

 American forms, the first and second being P. trivolvis and the 

 third P. amnion. 



1. Utah Lake, U. T., elevation 4498'.5 feet. Greater diameter 

 1*41 ; lesser, 1"04 inches. Long, of aperture "71 ; longitudinal 

 diameter of whorl at juncture of parietal callus -45 inch. 



2. Washoe Lake, Nevada, elevation 5006 feet. Greater diameter 

 1-30 ; lesser, 1"01 inches. Long, of aperture '60; long, of whorl at 

 junction of parietal callus •47 inch. 



3. Salinas Yalley, Cal., elevation 100 — feet. Greater diameter 

 1*24; lesser, '98 inch. Long, of aperture "90; long, diameter of 

 whorl at junction of parietal callus '55 inch. 



4. Clear Lake, Cal., elevation 1323 feet. Greater diameter 1-05 ; 

 lesser, -74 inch. Long, of aperture "77 ; long, diameter at junction 

 of parietal callus "76 inch. 



This last (4) is a typical Traskii. A comparison of the meas- 



