WoL. 1) Variation in Shells. 425 
the most interesting contributions to our knowledge that has been 
offered in many years. With the small number of species and indi- 
viduals found over this area, we cannot look for many varieties. A 
few, however, occur, and I now offer description of two varieties of 
Anodons, and add some notes and discuss the probable origin of 
our most and best marked species. 
Anodonta angulata var. subangulata (plate x, figs. 1 and 2). In 
general outline this form recalls at once its nearest relative, 4. angu- 
lata Lea. Compared with that species, it is more oval in outline; 
the ridge or angle radiating from the umbones to the anterior ventral 
margin of the shell is nearly or quite obsolete in the specimens before 
me; the anterior margin of the shell is more produced on its-dorsal 
edge, while its ventral edge is more rounded at the point where the 
angle terminates, and the intervening space between the angle and 
front margin of the shell is less concave in form. The shell is also 
less pointed and more rounded in its posterior outline, and is much 
less inflated in general form. In color and lines of growth the two 
shells agree fairly well. 
Compared with Anodonta Oregonensis Lea., its next nearest rela- 
tive, one variety is just the reverse in outline. It is broader in front 
from its dorsal to its ventral margin, and narrower behind, while 
A. Oregonensis is narrower in front from its dorsal to its ventral 
margin and broader behind. A. Oregonensis is also much more 
inflated, and some of its varieties inclined to be cylindrical, while 
our variety is flat and compressed. 
Habitat: Russian River, Putah Creek and Upper San Joaquin 
River, California. 
Length, from the anterior to posterior end, 334 inches. Height, 
from the dorsal to the ventral margin, 134 inches. Breadth, between 
valves, % inch. go 
From the examination and study of quite a number of specimens 
of both A. angulata and A. Oregonensis, 1 am strongly impressed 
with the idea that the former has evolved from the latter, although 
there is considerable difference in the form of the shells as well as in 
the habits of the two animals; but when the streams and lakes in the 
northern part of our State are thoroughly explored, I feel confident — 
these gaps will be closed. | 4 
Anodonta Oregonensis is a pond or lake, and not a river mussel, 
although it is sometimes found in our rivers. In ponds and lakes, 
