BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 1 47 



A study of the distribution charts shows us that, whereas a considerable number of 

 our local lamellibranchs are of very general distribution throughout Vineyard Sound 

 and Buzzards Bay, a )'et greater number show definite restrictions in relation either to 

 the character of the bottom or to temperature. The part played by the bottom in 

 determining the wealth of lamellibranch life is indicated to some extent in the figures 

 representing the average number of species per dredge haul taken upon the three chief 

 types of bottom. These are: Gravel and stones, j.-j; sand, 9.8; mud, ii.o. 



These figures are quite in accord with those giving the average number of species 

 per dredge haul in the Sound and the Bay : 



Vineyard Sound: 



Fish Hawk 8. 2 



Phalarope 7. 5 



Buzzards Bay: 



Fish Hawk 11. 5 



Phalarope 11. 6 



It is not evident, however, why the Phalarope stations of the Bay, which, on the whole 

 were decidedly less muddy than the Fish Hawk stations, should none the less show a 

 larger number of species. 



The lists of "prevalent" species for the three types of bottom (i. e., those present at 

 one-fourth or more of the stations) display a degree of uniformity which was unexpected 

 in view of the above shown differences in the wealth of species per dredge haul. The 

 number of prevalent species (16) is the same for sandy as for muddy bottoms, while 13 

 such species are listed for bottoms of gravel and stones. Of these, 9 are common to the 

 three lists. 



Passing to a consideration of the charts (122-160) we find a considerable variety 

 among the distribution patterns, but it seems possible to reduce these to comparatively 

 few types. These last are not, however, to be distinguished sharply from one another. 



Of general distribution. 

 Anomia simplex. 



Pecten gibbus borealis (scarce, however, in center of Bay). 

 Area transversa. 

 Nucula proxima. 

 Cardium pinnulatum. 

 Callocardia morrhuana. 

 Tellina tenera. 

 Ensis directus. 

 Clidiophora gouldiana. 



General in the Sound; common in the Bay, but restricted to inshore stations. 



Crassinella mactracea. 



Divaricella quadrisulcata (only 20 stations altogether). 



Cumingia tellinoides (not exactly general in Sound, and some records for middle of Bay). 



Spisula solidissima (some records for middle of Bay). 



Cochlodesma leanum. 



Corbula contracta. 



General in the Sound; in the Bay, restricted to lower half. 

 Mytilus edulis. 

 Astarte castanea. 

 Petricola pholadiformis. 



