PLANORBIS, 385 
This is the reverse of the var. juvenilis, the whorls increasing more in diameter and 
less in height, and thus exaggerating the difference which distinguishes the adult from 
_ the young form. It approaches P. caribeus, differing from it in the comparatively 
more inflated form and in the shape of the aperture. Perhaps an individual 
variation. 
Var. uhdei, n. (Tab. XXI. fig. 2.) 
Planorbis tenuis, var., v. Mart. in Malak. Blatt. xii. p. 56 (1865) *. 
Centre of the shell placed nearer the right side, this side thus being less excavated; the visible part of the 
penultimate whorl larger, occupying, together with the interior whorls, nearly one-half the diameter ; 
the left side, on the contrary, very deeply excavated, not angulated. Aperture large, rounded. 
Hab. Cuntrau Mexico (Uhde, in Mus. Berol."). 
Probably an individual variation, or perhaps an abnormally formed specimen, the 
excavation of the left side being somewhat irregular. 
Var. ecaggeratus, 0. 
Planorbis tenuis, var., Pilsbry, Proc. Acad. Phil. 1891, p. 322, t. 15. fig. 4°. 
Very inflated, the height of the aperture equal to the greater diameter of the whole whorl; last whorl 
somewhat angulated in its greatest periphery, especially near the aperture. 
Diam. maj. 144, alt. 143; apert. diam. 10 millim. 
Hab. Cxuntrat Mexico: Lake Patzcuaro (Heilprin 1). 
At first sight this seems to be an abnormal form; but as it appears to be confined 
to one particular lake, where no other form of this polymorphous species has been 
found, it may be a local variety, analogous to the lacustrine varieties of some of our 
European freshwater shells. 
Var. strebelianus. . 
? Planorbis trivolvis or corpulentus, Strebel, Beitr. Mex. Land- und Siissw.-Conch. i. p. 39, t. 5. 
fig. 19 (1873) * *. 
* Through the kindness of the officials of the Hamburg Museum I have been enabled to examine eight 
original specimens of this form from Strebel’s collection. They chiefly differ from normal P. tenuis in having 
the whorls increasing somewhat more gradually, so that, especially in the left side, the penultimate whorls 
occupy @ larger space, nearly but not quite one-half of the larger diameter. In two of these specimens the 
angular line of the penultimate whorls in the right side approaches and falls into the suture, thus causing the 
right side to appear funnel-shaped, the walls falling inward regularly from all sides; while in the normal 
P. tenuis the angular line is somewhat distant from the suture, and the appearance of the right side is that of 
a thick string coiled spirally inward. Both these characters are very well shown in Strebel’s figure, but 
among his own specimens there are clear transitions to the normal form of P. tenwis; as regards the second 
character, they are even in the majority (6:2). Strebel’s figures 19 (strebelianus) and 21 (tenuis) are taken 
from the most extreme forms, and the very evident difference is exaggerated by the circumstance that the 
figure 21 represents a not quite fully-grown specimen, according to the form of the aperture, and that the 
angular spiral lines of the whorls on the left and right sides are scarcely or not at allexpressed. From certain 
BIOL. CENTR.-AMER., Terr. and Fluviat. Mollusca, Apri] 1899. 49 
