352 A Monograph of the Species of Pisidium. 
inflated ; the beaks are larger and more tumid ; it is also a 
much smaller shell. Say describes the Cyclas dubia as being 
six twentieths of an inch in length; the P. variabile is only 
four twentieths of an inch in length ; and that it is a full- 
grown shell, I am led to believe not only from its heavy 
striations and mature appearance in general, but also from 
having found young in the shell. 'The young is not so oblique 
as the adult; it is more elongated, less inflated, and of a light 
yellow color. As a general rule, the coloring of this species 
varies much in different localities. The specimens collected 
from Rowley, Essex County, Mass. are larger than any I 
have seen from other places; their color is also lighter. The 
animal is remarkable for its sluggishness and want of activity. 
This is one of our most common New England species, being 
found in nearly every stream, and at all seasons of the year, 
though most plentifully during the spring. 
um 
2. Pistp1om Apamsi Nobis. 
Cabinet of the B. S. N. H. 
PLXL Fig. 1, 2; 3. 
me nilida Apams et Micugrs. Bost. Journ. Nat. Hist. iv. 39, pl. 4. 
i Adamsii Nosis. Stimpson, N. Engl. Moll. 16. (P. nitidim 
preoc. ) 
Description. Shell subovate, full, solid, oblique; margins 
moderately rounded; beaks small, tumid ; surface smooth, 
lines of growth light, deeper towards the margins; color 
light gray, with a zone of reddish yellow extending round the 
outline of the margins; in some cases the zone is wanting 
de "d the shell is of a uniform gray ; interior whitish ; hinge 
