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 hardiv 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 Aid. 

 Strobilops labyrinthica Say. 

 Succinea ovalis Gld. 

 Carychium exiguum Say. 

 Limnsea stagnalis L. 

 Limnsea desidosa Say. 

 Limnsea humilis Say. 

 Physa ancillaria Say. 

 Physa heterostropha Say. 

 Physa integra Hald. 

 Planorbis deflectus Say. 

 Planorbis parvus Say. 

 Planorbis bicarinatus Say. 

 Planorbis trivolvis Say. 

 Planorbis campanulatus Say. 

 Ancylus rivularis Say. 

 Valvata tricarinata Say. 

 Amnicola limosa Say. 



Amnicola lustrica Pils. 

 Campeloma integra Say. 

 Carcpeloma obesa Lewis. 

 Campeloma subsolidum Anth. 

 Pleurocera elevatum Say. 

 Goniobasis livescens Mke. 

 Unio ventricosus Bar. 

 Unio spatulatus Lea. 

 Unio uovi-eboraci Lea. 

 Unio pressus Lea. 

 Margaritina rugosa Bar. 

 Margaritina deltoidea Lea. 

 Auodonta subcylindracea Lea. 

 Anodonta Footiana Lea. 

 Spbreriuru simile Say. 

 Sphrerium striatinum Lam. 

 Pisidium compressum Prime. 

 Pisidium sp. 



