BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 69 
AMPHINEURA: GastTRopopa—Continued. 
Bittium alternatum (37). 
Littorina litorea (54). 
Crepidula fornicata (72). 
Crepidula convexa (32). 
Chzetopleura apiculata (23). 
GASTROPODA: 
Busycon canaliculatum (24). 
Tritia trivittata (85). 
Anachis avara (64). 
Astyris lunata (54). 
Eupleura caudata (37). 
Urosalpinx cinereus (39). 
Crepidula plana (58). 
Polynices duplicata (36). 
Polynices triseriata (30). 
TUNICATA: 
Didemnum lJutarium (27). 
The total number of species in the foregoing list (54) is exactly the same as that 
contained in the one immediately preceding it. In fact there has been a rather striking 
uniformity in the numbers comprised in these lists, ranging as they do from 46 to 55. 
Of the 54 species in the foregoing table, 41 (76 per cent) are common to this and to the 
list of Fish Hawk species in Buzzards Bay. On the other hand, a number not much 
inferior to this (37=69 per cent“) are common to the present list and to that given 
for the Phalarope stations of Vineyard Sound, among the latter being some which are 
not recorded in the other Buzzards Bay list. A few others in this list are only found 
elsewhere in the Fish Hawk list for Vineyard Sound. 
While, therefore, the Phalarope list for Buzzards Bay resembles the Fish Hawk list 
for Buzzards Bay more closely than any of the others, it must be pointed out that it 
contains a considerable number of species which are prevalent throughout the Sound, 
but which in the Bay are to be found only at the inshore dredging stations. This fact, 
which is not very strikingly illustrated by these figures, will appear much more clearly 
when the charts portraying the distribution patterns of certain species are scrutinized. 
Tables have likewise been prepared listing the ‘‘prevalent’’ species for each type of 
bottom. The same criterion has here been employed of admitting only those species 
which have been encountered at one-fourth or more of the number of stations belonging 
to the group in question. 
After considerable thought the following classification of bottoms has been adopted 
for present purposes, not as being an ideal one, but as being the most simple one possible 
consistent with a fair regard for accuracy. ‘The only strictly exact classification would 
recognize as many types of bottom as there are combinations of ingredients listed; but 
such a classification would be altogether too cumbersome for the purposes of our statis- 
tical treatment. We realize that the grouping here employed must result in a quite 
inadequate characterization of the habitat of many species. A specimen may ostensibly 
have come from a muddy or a sandy bottom, when, in reality, it was growing attached 
to a shell or other solid object. We have, nevertheless, included as muddy and sandy 
those bottoms in which shells were likewise recorded. This has been done for the reason 
that shells or fragments of these were scarcely ever wholly lacking from the contents 
of the dredge. Again, certain living mollusks which move freely over the bottom afford 
support for attached organisms just as well as do dead shells. Surely the presence of 
such should not suffice to constitute a ‘‘shelly’’ bottom. The same may be said regard- 
ing shells occupied by hermit crabs, which abound throughout the entire region, giving 
support to hydroids, Bryozoa, barnacles, Crepidule of several species, and other 
organisms. 
@ Only ss per cent of the Fish Hawk list for Buzzards Bay were common to the Fish Hawk list for Vineyard Sound. 
