352 



colors, but the majority are pale. Altogether there are 

 to be found among these deep-water forms "innumerable 

 illustrations of beauty, adaptation, or unusual character- 

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

 other band, the thick-shelled pelecypods predominate, and 

 this is especially true of the shore zone. 



Pelecypods, like brachiopods, are excellent facies indica- 

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

 species, or at least the fauna! combinations, are dependent 

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

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

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

 foreign objects — either by direct cementation or by a bvssus. 

 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 (Peeten, 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. E\ fi a 

 Solen, though normally a burrowing animal, will swim for 

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

 often be seen circling around in an aquarium 1 , by a series of 

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

 siphons. A number of pelecypods bore into wood or stone 

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

 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. Boring sponges will 

 riddle the shells of littoral species, and corallines. ihyozoa, 

 worms, and hydroids will attach themselves to the shells. 

 There is abundant evidence in the riddled and punctured 

 shells, that the Palaeozoic molluscs were subject to similar 



