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 Scrobicularia. 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 Myidae (Plate XV., Fig. 4) and Solenidse (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 (i.e., invested by 

 the mantle) : Chlamydoconcha, which passes its life 

 attached to the sheltered sides of rocks by its byssus ; 



