BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 1 73 



habitats and regions are of approximately equal length, the number of species ranging, 

 with a single exception, between 50 and 55. These figures, of course, in no way express 

 the relative wealth of animal life in these situations, this last being dependent upon the 

 number of individuals rather than the number of species. Unfortunately we have no 

 data sufficient for the purpose of giving a statistical expression to the real wealth of life 

 upon different portions of the local sea floor. Particular spots were found, it is true, 

 which were in large degree destitute of life, but whether or not any one of the types of 

 bottom or of the larger subdivisions of our region was more densely populated than 

 any other can not be stated with mathematical certainty. It is our general impression 

 that living organisms were found to be somewhat less abundant upon bottoms of com- 

 paratively pure sand, although it is true that this is the prevailing type of bottom in the 

 western portion of Vineyard Sound, to which many of our species are restricted." 



Another fact which may be regarded as surprising, despite the differences pointed 

 out above, is the comparatively small proportion of the species which are restricted to 

 any particular type of bottom. Thirty species are common to all three of the lists 

 which give the prevalent forms for each type of bottom, this number representing, on the 

 average, 60 per cent of the number contained in each list. But even this figure does not 

 fairly express the number of those which were actually found with considerable fre- 

 quency upon all three types of bottom, since each list is restricted to species so common 

 as to have been encountered at one-fourth of a given group of stations. Again, only 

 26 per cent of the species contained in the list of prevalent mud-dwelling forms is pecul- 

 iar to that list ; while only 24 per cent of the list for bottoms of stones and gravel, and 

 only 13 per cent of that for sandy bottoms are peculiar to their respective lists. 



We do not think that these figures fairly express, -however, the obvious differences 

 in the characteristic faunal aggregations for different types of bottom, as presented to 

 the eye. This is because they do not take into account the relative number of indi- 

 viduals belonging to the various species. Certain species which are characteristic of 

 muddy bottoms (e. g., certain bivalve mollusks and worms) are present in great numbers 

 in an average dredge haul made upon such a bottom. But along with them are smaller 

 numbers of a great variety of species, which are not especially characteristic. The same 

 may be said of the other types of bottom. Thus the real distinctness of the faunal 

 aggregations in question could only be adequately expressed by reference to the relative 

 abundance of each species. 6 Again it must be once more emphasized that the mixing 

 up, in a single dredge haul, of organisms from several quite distinct bottoms is in some 

 measure responsible for this apparent lack of distinctness in their respective habitats. 

 This is particularly true of relatively small areas of sea floor, such as those under consid- 

 eration, in which quite various deposits are found to alternate with one another at fre- 

 quent intervals. It is likely, indeed, that under such circumstances there is much over- 

 lapping and intermingling of faunal aggregations which elsewhere might be far more 

 distinct. Finally, it must be remembered that the lists of "prevalent" species, as here 

 constituted, exclude many forms which are highly characteristic of the bottoms in ques- 

 tion, and which, in some cases, are restricted to them. 



o It is here, indeed, that line fishing for mackerel and flounders is carried on with the greatest success. 



& Of course, in a certain measure the wealth ol a given species in individuals determines the frequency with which it appears 

 in the dredging records. It is self-evident that the more abundant species are more likely to be taken than less common ones. 



