232 



NATURE OF COAST. 



gion there have been repeated interchanges of 

 fresh, brackish, and salt waters, and the results are 

 shown in a remarkable manner in the genera Palu- 

 dina, Melanopsis, and Neretina. These genera pre- 

 sent three series of peculiar forms, so different, 

 a that at first examination we appear to have before 

 us very distinct and well-marked species." He was 

 satisfied, however, " that they were the same species, 

 assuming Protean variations," under varying con- 

 ditions of the medium in which they lived, 



The nature of a coast-line, and the composition 

 of the deposits which form the sea-bed at different 

 zones of depth, are conditions which exercise an im- 

 portant influence on the general character and 

 abundance of marine life. 



The Testacea that live attached, or which perfo- 

 rate cavities for themselves, require hard strata : 

 on rocky and stony coasts Mytilus, Chiton, Patella, 

 Haliotis, Gyprcea, and others are found. Some 

 boring shells require or prefer limestone recks, such 

 as Gastrochcena, Saxicava rugosa, and Lithodomus. 

 Others, like the Pholades, are as often found in 

 pure sand-stones. 



The amount of weed in the upper sea-zone de- 

 termines the numbers of the Phasianellw, Pissoce, 

 Lacunae, and Littorince which a fauna will have. 

 Granitic coasts, or those of hard slates or sand- 

 stones, seem to afford attachment for a greater 

 quantity of marine vegetation than do limestone 



