174 BULLETIN OF THE BUREAU OF FISHERIES. 



It would be hard to characterize in any brief statement the faunal assemblages 

 proper to the various types of bottom. Such assemblages have been presented in 

 four illustrative cases (p. 58-62), and composite pictures, including the more charac- 

 teristic species, have been given elsewhere in chapter iii. An attempt to still further 

 condense these data would, we fear, result in a mere statement of platitudes. It may be 

 allowable to mention, however, that the most characteristic species found upon muddy 

 bottoms were annelids and bivalve mollusks, many of which were restricted to such 

 bottoms; the most characteristic species found upon bottoms of stones or gravel were 

 hydroids, Bryozoa, and ascidians; while the few species which were in any real sense 

 restricted to bottoms of clear sand were either burrowing species {Ovalipes, Echinarachnius , 

 certain lamellibranchs) , or fishes (flounders and skates) which adhered closely to the 



bottom. 



3. THE INFLUENCE OF TEMPEt^TURE. 



The temperature factor is, with little doubt, the controlling one in the case of many 

 species belonging to several different phyla. On page 74 is given a list of species which 

 were dredged predominantly or exclusively in the colder waters of the region, i. e., at 

 the western end of Vineyard Sound and the mouth of Buzzards Bay. Here the summer 

 temperature of the bottom water averages about 10° F. (5.6° C.) lower than in the imme- 

 diate neighborhood of Woods Hole. Reference to the stated ranges of these species 

 revealed the fact that in 15 out of the 20 cases they are predominantly northward-ranging 

 forms, some of which, indeed, are near their southern limit of distribution. Reference 

 has also been made to a number of less common forms having a similar distribution, 

 but which are not included among those for which distribution charts have been pre- 

 pared. This large proportion of northward-ranging species among those occupying the 

 colder waters of Vineyard Sound and Buzzards Bay is significant in view of the fact 

 that a decided minority (23 per cent) of the species dredged by us with any frequency 

 throughout the region at large are to be classed as northward-ranging, according to the 

 standard employed. 



It is of interest, also, to note that a large proportion of these colder water species 

 were likewise taken by us at Crab Ledge, off Chatham, where the water temperatures in 

 summer are even lower than at the mouth of Vineyard Sound. At Crab Ledge and at 

 certain other outlying points were also taken a considerable number of species which 

 appear never to enter Vineyard Sound or Buzzards Bay at all. So far as we have 

 ascertained the ranges of the species, they belong, almost without exception, to the 

 "Acadian" fauna characteristic of the waters north of Cape Cod. 



Another list was presented (p. 76) of species which, though otherwise of general 

 distribution throughout Vineyard Sound, and in many cases throughout Buzzards Bay 

 as well, are absent from just those waters to which the northern types are restricted. 

 This list was found to include none of the strictly northern tj'pes, while more than half 

 of the species there included were forms which found in Cape Cod their northern limit 

 of distribution. It is probable that the temperature factor is the one responsible for 

 this type of distribution in some cases at least. Many of these species, it is significant 

 to state, are conspicuously absent from Crab Ledge. On the other hand, it is hkely that for 

 some other organisms (e. g., the ascidians) the uniformly sandy condition of the bottom 

 in this outer portion of Vineyard Sound and the scarcity of solid objects suitable for 

 attachment render it an unfavorable habitat. 



