352 



colors, but the majority are pale. Altog-ether there are 

 to be found amoiig; these deep-water forms "'innumerable 

 illnstrations of beautv, adaptation, or unusual character- 

 istics. . . .'' ( Agassiz.) In the littoral district, on the 

 other hand, the thick-shelled pelecvpods predominate, and 

 this is especially true of the shore zone. 



Pelecvpods, like brachiopods, are excellent facies indica- 

 tors, for, though thej live on all kinds of sea bottom, the 

 species, or at least the faunal combinations, are dependent 

 on, and characteristic of, the particular facies on which they 

 live. The majority of pelecypocls are free animals, a few, 

 such as the oyster, mussel, and the like, being attached to 

 foreign objects — either l)y direct cementation or by a byssus. 

 The free pelecypods have frequently the power of locomo- 

 tion, Unio Mactra and others traveling occasionally for con- 

 siderable distances. Generally, however, these molluscs lie 

 buried wholly or partially in the sand, and never change 

 their- location except when disturbed by storm waves. Some 

 few pelecypods ( Pecten, Lima) have the power of swimming- 

 short distances by the opening and closing, in rapid succes- 

 sion, of their valves, and the forcible ejection of water. Even 

 Solen, though normally a burrowing animal, will swim for 

 some distances in search of the proper bottom, and it may 

 often be seen cii'cling around in an aquarium, by a series of 

 jerks, due to the periodic ejection of the water from the 

 siphons. A nundjer of pelecypods bore into wood or stone 

 (Teredo, Lithodomus, Saxicava, etc.), leading a sedentary 

 life within the habitation thus formed. 



The bivalve molluscs have many enemies which prey upon 

 them. Not the least of these are the carnivorous gastro- 

 pods, whose depredations are usually marked by the vast 

 number of shells with round holes bored into them which 

 are scattered along our beaches. Boi-ing sponges will 

 riddle the shells of littoral species, and corallines, Bryozoa, 

 worms, and hyclroids will attach themselves to the shells. 

 There is abundant evidence in the riddled and punctured 

 shells, that the Palaeozoic molluscs were subject to similar 



