414 THE CHICAGO ACADEMY OF SCIENCES. 
Ecotocy: “The specimens in this lake (Mud Lake) are small, 
light horn-colored, sometimes ornamented with revolving bands; the 
color of the animal varies. The specimens at the water’s edge are 
bluish-black or gray, and those in deep water are very light and some- 
times orange. Specimens are most common in water about three 
feet in depth, on a gravel bottom, where they feed on conferve, grow- 
ing on the rocks. In square lake it is always found at low water mark, 
on gravelly or stony bottom, or on a gravelly bar where there is a 
strong current from an inlet or thoroughfare.” (Nylander, Maine.) 
It is an interesting fact that the large variety mighelsi is found 
in the larger bodies of water, while the typical emarginata occurs 
usually in or near small thoroughfares or bayous. : 
“In a small inlet (Tonkin Bay) which opens to the east upon the 
lake, with steep, approximately parallel sides. This inlet is narrowed 
half way up by two beaches which reduce the wave action in the inlet. 
In this inlet Lymne@a emarginata lives along both sides where the bot- 
tom is rock, but not across the ends. It lives only on a rock substratum, 
which may be either horizontal or vertical, and in water up to 45 centi- 
meters in depth. In Conglomerate Bay emarginata lives in water in 
15-45 cm. deep, in the deeper water on the tops of flat rocks, in the 
shallower water, also, on the vertical sides and in small crevices. They 
never occur on the sand or gravel deposited around the rocks, as is fre- 
quently the case near the sand beach at the upper end of the bay. 
“In connection with the rock beaches may be mentioned the beach 
pools, which are depressions in the rock filled with water by high 
waves. They are naturally most abundant on flat or gently sloping 
beaches, and their permanency varies with their size and depth, affect- 
ing evaporation, and with their height above the lake, affecting the 
frequency with which they are filled. In those which are permanent 
are found shells, Limnea emarginata Say and Planorbis parvus Say. 
(Gleason, Isle Royale). 
REMARKS: On page 69 of his Alaska Mollusks, Dr. Dall says: 
“After considerable study I have been forced to the conclusion that 
several species were identified under this name by Say himself, as well 
as others.” Dr. Dall is right in this statement, excepting that I be- 
lieve Say understood his species pretty thoroughly. There is no doubt 
that later students have mixed the true emarginata with forms of 
catascopium and palustris, besides confounding several varieties under 
this name. The original specimens came from Maine and it is to this 
state that we must look for typical emarginata. Such material has been 
abundantly collected by Mr. Olof O. Nylander in the lakes of Aroos- 
