1893.] NATURAL SCIENCES OF PHILADELPHIA. 243 



trary in V. altilis (Man. Conch., PI. 32, fig. 17), where the aperture 

 is also very little inclined. 



As to the shape of the aperture, there are two principal groups of 

 species — in one it is nearly circular, no wider than high or rarely 

 so ; in the other the transverse diameter is greatest, and the aperture 

 is ovoid or pear-shaped. Most forms of the latter group have no 

 thickened lip, as a rule, while those of the former may be of the 

 same character, or provided with a more or less strong white lip. 

 In a few forms the peristome is simple and straight, as in V. pollin- 

 ensis and V. altilis, while in most it is everted (or " reflexed ") in 

 various degrees and rather abruptly, as in pulchella, costata, etc., or 

 more gradually as in cyclophorella. It must be specially noted that 

 in all instances the peristome is straight above, not everted, at the 

 suture and for a greater or less distance from it. This will not be 

 repeated as a general character in the special descriptions. 



It remains to say a few words in general about the structure of the 

 surface of the shell. Ribbed and plain species have been distin- 

 guished, but there are all possible intermediate forms between these 

 extremes, and it may be added at once that even in the same species 

 considerable variation may be found, e. g., in V- pulchella and cos- 

 tata. What we generally call ribs are membranous elevations or 

 duplicatures of the epiconch, and, properly speaking, they do not 

 deserve that name. They vary from very coarse and rather distant, 

 and equidistant, as in some forms of V. costata, to quite fine and 

 crowded, and more or less irregularly set. I counted from 22 (F. 

 costata var. amurensis) up to nearly 70 (in V- cxjclophorella) on the 

 last whorl. These membranous ribs generally stand on stronger 

 stripe or fine ribs of the shell itself. But in some species there are 

 rather strong true ribs devoid of membranes, as in V. gracilicosta 

 Reinh. To what degree the variability may go is seen in V. costata 

 var. helvetica, which has not a trace of membranous ribs and the 

 "shell ribs" are very fine striae, visible only with a good lens. It 

 is not sufficient, then, to say in descriptions, that a shell is " ribbed " 

 or " not ribbed," but the character of the ribs must clearly be 

 stated, as this is a prominent character. On the dead shells, and 

 sometimes partly on the living, the epiconch loosens and becomes 

 lost by the action of the water or atmospheric influences, and with 

 it disappear the membranous ribs. Such specimens, mixed with 

 more or less perfect ones, are especially found in drift materials 

 along rivers and streams. It is unnecessary to say that fossil shells 



