1881.] NATURAL SCIENCES OF PHILADELPHIA. 97 



acid in a saucer or watch-glass), until the shell is eaten off to a 

 line which obliterates the umbilicus, when the nucleus of the spire, 

 the tip, and half a whorl to a whorl and a-half will be found re- 

 maining. 



Those who insist on the dextral character of the shells in 

 Planorhis, unless the}' except the species I have named, are thus 

 compelled to demonstrate how in the sequence of growth the 

 umbilicus can precede the nucleus. 



Though specimens of the forms under discussion, in various 

 embryonic stages, have frequently excited my attention, j-et the 

 material, so far as adolescence is connected with the present line 

 of inquiiy, was at the moment, unfortvinatel}^, inaccessible. I 

 have therefore been compelled, in order to present such structural 

 features of the shells as are related to the direction (right or left) 

 of the volutions, the form of the aperture, etc., to use adult speci- 

 mens, and by breaking back, piece by piece, and whorl after 

 whorl, towards the nucleus, until the larger whorls are sufflcientl}^ 

 removed, so that the apex or spire ceases to be either concave or 

 depressed, and is simply flat. It would be almost, if not quite 

 impossible to do this with the smaller species, owing to their 

 diminutive size and exceeding fragility, and difficult to obtain the 

 necessar}^ sections for illustration herein, by the use of acid. 



The figures (10) are drawn from specimens of Planorhis coiyu- 

 lentus collected in Oregon, also in Clear Lake, 

 Fig. 10. California, by that indefatigable collector, Mr. 



C. D. Yoy. Before manipulation they measured 

 as follows : 

 Plan, corpuientus- Largest diameter, . . -94 inch. 



nuclear whorls. Height. . . . • . "38 inch. 



Number of whorls, four and a-half, 



which were broken back to one and a-half whorls, with a diameter 

 of "10 inch. ; height -15 inch. The umbilicus in one instance was 

 still discernible — in the others, destroj^ed. This species is widely 

 distributed and occupies an extensive geographical area, on the 

 western side of the continent, from the Columbia River in the 

 north ; easterly to Lake Winnipeg ; and southerly to Cape St. 

 Lucas. ^ Binney says, " F. corpulentus is catalogued from Guate- 

 mala b}^ Mr. Tristram," 



^ Prof. Geo. Davidson collected specimens at this place. 



