BIVALVES. 
39 
like Gyclas lacustris. Length three lines, thick¬ 
ness one and a half, and width two and a half. 
In ditches and slow streams widely diffused. 
Pisxdium pulchellum (PL IV., fig. 24) is the 
smallest of the genus, differs from the last in 
size, and is of a less triangular form. The 
shell is only one and a half lines long and wide, 
half a line thick, of a glossy white, sometimes 
greyish, finely and irregularly striated. 
This handsome and well-marked species well 
merits its specific name ( pulchellus , small and 
beautiful) ; it is universally distributed. It in¬ 
habits stagnant and running water, and at the 
same time and place may be found on submerged 
plants, and buried in the mud. It occurs in the 
newer Tertiaries. 
Professor Macgillivray made the following 
observations on individuals of this species found 
in a ditch near Aberdeen When advancing 
in the water, the animal opens its valves a little, 
places itself erect by means of the foot, which 
it gradually protrudes until it extends to a length 
and a half of the shell, but often to twice its 
length. When thus extended it is of a linear- 
oblong form, very little flattened, narrowed but 
rounder at the end. It then contracts, and 
drags the shell quickly forward; after which it 
is again extended, and again contracts. It is 
not always stretched out in a direct line, but is 
