1893.] NATURAL SCIENCES OF PHILADELPHIA. 241 



3*5 mm.; all are openly and rather widely umbilicated ; the number 

 of whorls varies only between 3 and 4J at most; the coloration is 

 from colorless to pale or reddish horn, or to light gray. Yet there 

 are some features likely to sufficiently characterize the different 

 forms, even better than are those of most groups of Helve with much 

 larger shells. The umbilicus may be rather narrow from its begin- 

 ning, or very wide, distinctly showing all the volutions, as in V. per- 

 spective!, and some varieties of costata. Usually it widens consider- 

 ably and rather abruptly, with the last b-% whorl receding to the 

 periphery below ; sometimes this receding 'is not effected in the 

 part of the last whorl close to the aperture, but earlier, to keep in 

 a more regular circular or spiral direction in the last part, or even 

 to turn inward and thus again narrow the umbilicus; it is thus in 

 V. cyclophorella and tenuilabris (i. e., the Kroellwitz shell, Man. 

 Conch., PI. 33, fig. 31) for instance. The peripheral outlines of the 

 shell, as well as the umbilicus, sometimes are distinctly elongate in 

 direction, not of the larger diameter, but of one cutting off a part of 

 the last whorl (Man. Conch., PI. 33, fig. 35). There is also in the 

 species named and in V. costata var. pyrenaica, a slight but dis- 

 tinctly marked obtuse angle in the peripheral outline, about i of a 

 volution above the aperture (Man. Conch., fig. 30, 34). In V. exeentrica 

 the umbilicus is narrowed by the last whorl, and then widened (tig. 8) 

 while above, the aspect of the shell is quite different from that ,of 

 the species mentioned (Man. Conch., PI. 33, figs. 32-35). These 

 characters may appear trifling, but they essentially characterize the 

 different forms, and must impress themselves upon any one examin- 

 ing and comparing great numbers of Vallonia. In comparatively 

 few forms the umbilicus shows a regular or approximately regular 

 spiral, as in V. declivis (and altilis), and also in some varieties of 

 costata (helvetica). 



As has been pointed out by different authors, the last whorl is often 

 decurved or descends toward the aperture, and that in the majority 

 of species. But it does so in different degrees and ways which are 

 very characteristic and deserve our special attention. 1. The lateral 

 i-i whorl may simply, gradually and steadily descend, without a 

 marked curvature of the middle upper part or " back," as it will be 

 called hereafter, of the whorl, and without a deepening of the 

 suture, such as is seen in P. declivis (Man. Conch. PI. 32, figs. 13, 

 16, 17), and in V. pollinensis Paul, (from the description). 2. In 

 other forms only the suture descends or deepens, rather shortly and 



