VARIATION IN FORM. 
6.3 
bed of the river or pond, the efifects of the special features of their 
environineiit are shown more particularly at the posterior or siphonal 
end of the shell, which is fully exposed to such modifying influences 
as may exist. In still pools, lakes, or slowly flowing waters the shells 
of these genera may be more or less equally inflated, resembling in 
this respect those species which do not partially imbed their shells 
in the muddy bottom of the water. In rapidly flowing streams and 
rivers the shells are often specially characterized by a remarkable 
compression and extension of the posterior end, as in Unio pictorum 
var. platyrliinclioidea Dupuy — of which Dr. Jeffreys’ var. compress^a 
is a sub-variety of somewhat broader form and slightly less sinuate 
lower margin — this remarkable variety was originally discovered in 
Fig. 136 . — Unio pictonou var. platythinchoidea Dupuy, 
Hethersett Lake, near Norwich, collected by Mr, A. G. Stubb.s. 
Showing a compressed burrowing Biv'alv’e, usually characteristic of flowing waters. 
this country by Mr. Bridgman in the River Wensum, near Norwich, 
which is a slow and sluggish stream with a layer of soft mud over a 
hard bottom ; the particular parts inhabited by this peculiar variety 
are the places where the river bends sharply and forms what are 
locally called “horse-shoe reaches,” the current rushing rather 
sharply at the last bend to the opposite bank and forming an eddy 
next the shore, the rush of the current removing all the loose 
soft mud covering the hard bottom. These remarkable shells are 
found burrowing in the hard ground just inside and at the edge 
of the sharp current, next to the eddy, and are assumed with great 
probability to be an effect of the ru.sh of the current and the con- 
sequent washing away of the softer river bed, rendering it more 
difficult for the mollusk to imbed its shell, this process being, how- 
evei’, probably facilitated by the more blade-like character of the 
shell generally, the broader valves when firmly imbedded preventing 
the mollusk being washed from its position too easily, and as the 
thin dorsal posterior edge of the shell is presented to the current, 
there is less friction to overcome. It is, however, very remarkable 
