108 THE LIFE OF THE MOLLUSCA 
overlooked because it is situated at the back of the 
broad hinge-plate. 
In proportion, however, as the Bivalve seeks 
shelter from the strains of the shifting sand, either 
in quieter waters, or by burrowing deeper, so the 
shell in response tends to become less heavy and 
solid, and to assume a flatter shape, permitting of 
more rapid passage down into the silt. This is seen 
in the later date forms such as Tellina, Psammobia 
(Plate XV., Fig. 7) and Scrobiculavia. The habit of 
burrowing deeper is of necessity accompanied by an 
increase in the length of the siphons to insure proper 
respiration, and this in turn results in the prolonga- 
tion of the posterior portion of the test to house 
them, as well as the ultimate abandonment of the 
flattened form, till finally, in the deepest burrowers, 
the Myide (Plate XV., Fig. 4) and Solenidz (Plate 
XXX., Fig. 8) the closed shell is frankly abandoned, 
and the valves, which no longer cover the whole 
animal, function solely as fenders against lateral 
pressure from the surrounding silt. 
Facility in penetration is, probably, likewise the 
accountable cause of the elongate shape of the rock- 
-boring representatives of several families of Pele- 
cypods. 
There are a few instances among the Bivalves in 
which the shell becomes internal (7.¢., invested by 
the mantle): Chlamydoconcha, which passes its life 
attached to the sheltered sides of rocks by its byssus; 
