122 THE NAUTILUS. 
denly developed upon the shoulder of the last whorl parallel with 
superior carina and at about one-third of the distance between it 
and the suture. The interval between the two being a groove, rather 
than a continuation of the flat surface of the shoulder. In a some- 
what similar specimen there is no distinct carina developed, but 
simply a sharp edge, where the flattened surface of the shoulder is 
cut by the groove which encircles it just inside the superior carina. 
In two other examples, the suture of the last half of the body whorl 
is decidedly channelled, increasingly so as it approaches the aper- 
ture, with an indistinct elevation hardly sufficient to be termed a 
carina, bordering its outer edge. 
Although these variations were found in only half a dozen spec- 
imens out of a considerable number, it is nevertheless worthy of 
notice, that in each instance the growth of the shell had been entirely 
normal until about the beginning of the last whorl. Then suddenly 
and apparently after a period of rest, such as hibernation, with a 
renewal of growth the change of sculpture begins. This would 
naturally seem to be the result of some change in the environment. 
But, whatever was its source, it was not sufficient, evidently, to affect 
the whole colony. However, as some of the examples are not fully 
grown, it may be possibly attributed to the same cause, which very 
shortly exterminated them all. Unfortunately, what that was, is 
purely a matter of conjecture. 
The complete list of the species found is as follows: 
Zonites radiatulus Ald. Amnicola lustrica Pils. 
Strobilops labyrinthica Say. Campeloma integra Say. 
Succinea ovalis Gld. ~ Campeloma obesa Lewis. 
Carychium exiguum Say. Campeloma subsolidum Anth. 
Limneea stagnalis L. | Pleurocera elevatum Say. 
Limnsea desidosa Say. Goniobasis livescens Mke. 
Limnea humilis Say. Unio ventricosus Bar. 
Physa ancillaria Say. Unio spatulatus Lea. 
Physa heterostropha Say. Unio novi-eboraci Lea. 
Physa integra Hald. Unio pressus Lea. 
Planorbis deflectus Say. Margaritina rugosa Bar. 
Planorbis parvus Say. Margaritina deltoidea Lea. 
Planorbis bicarinatus Say. Anodonta subeylindracea Lea. 
Planorbis trivolvis Say. _ Anodonta Footiana Lea. 
Planorbis campanulatus Say. Spheerium simile Say. 
Ancylus rivularis Say. Spheerium striatinum Lam. 
Valvata tricarinata Say. | Pisidium compressum Prime. 
Amnicola limosa Say. Pisidium sp. 
