ioi6 PRINCIPLES OF STRATIGRAPHY 



monly characterized by exceeding delicacy of shell and sculpture, 

 the shell being- often quite transparent. Some deep-water species 

 show bright colors, but the majority are pale. Altogether there 

 are to be found among these deep-water species "innumerable illus- 

 trations of beauty, adaptation, or unusual characteristics . . ." 

 (Agassiz). In the littoral district, on the other hand, the thick- 

 shelled pelecypods predominate, and this is especially true of the 

 shore zone. 



Pelecypods, like brachiopods, are excellent facies indicators, 

 for, though they live on all kinds of sea bottom, the species, or 

 at least the faunal combinations, are dependent on, and character- 

 istic 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 to foreign objects — either by direct 

 cementation or by a byssus. The free pelecypods often have the 

 power of locomotion, Unio, Mactra, and others traveling occasion- 

 ally for considerable 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 pele- 

 cypods (Pecten, Lima) have the power of swimming short dis- 

 tances by the opening and closing, in rapid succession, of their 

 valves, and the forcible ejection of water. Even Solen, though 

 normally a burrowing animal, will swim for some distance in search 

 of the proper bottom, and it may often be seen circling around in 

 an aquarium, 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 

 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 gastropods, whose depre- 

 dations are usually marked by the vast number of shells with 

 round holes bored into them, scattered along our beaches. Boring 

 sponges will riddle the shells of littoral species, and corallines, 

 Bryozoa, worms, and hydroids will attach themselves to the shells. 

 There is abundant evidence in the riddled and punctured shells 

 that even the Palaeozoic molluscs were subject to similar attacks 

 of boring sponges and carnivorous gastropods. When the animals 

 die, their valves usually fall apart ; and from their position, and 

 the character and direction of the waves and currents, one valve 

 may be carried shoreward, the other, seaward. This explains the 

 frequent predominance, along the shore and in certain local por- 

 tions, of fossiliferous beds of one valve, the other being entirely 

 absent or at least very rare. 



