BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 15! 
as stones and shells. On the other hand, it is not improbable that the temperature 
factor has been partly responsible for the distribution of Chetopleura in Vineyard Sound, 
as in the case of a number of other southern species which appear to avoid the colder 
waters of the region. Like those which have previously been discussed, Chetopleura 
was not recorded by us from Crab Ledge. The range of this species, as stated by Dall, 
extends from Cape Cod to Haiti. Our region thus lies at or near its northernmost 
limit of distribution. 
III. GASTROPODA. 
Of the Gastropoda we have recorded 123 determined species, together with 10 
which were doubtful or undetermined. Sixty-four (+2?) of these species were encoun- 
tered during our Survey dredgings, and at least 17 are believed to have been previously 
unrecorded for the region. 
Verrill and Smith, in their Vineyard Sound report, listed 93 species, of which, 
however, only 65 were definitely recorded for specified points within the region, although 
the ranges of 20 more, as stated by them, would include the Woods Hole region. The 
completeness of Verrill’s list, as regards our more familiar species, renders conspicuous 
two exceptions. One is our now abundant periwinkle, Littorina litorea, which did not 
reach Woods Hole in its southward migration until the year 1875; the other is Lacuna 
puteola, an allied species though quite a minute one, which is likewise very common 
here at the present time. Whether or not this latter mollusk is also a comparatively 
recent immigrant can not be stated. It has long been known in the British Isles. 
In the case of the gastropods, as in that of the lamellibranchs, our list of species is 
greatly exceeded by all of the other faunal catalogues which have been summarized in 
our comparative table. The difference in favor of the Plymouth catalogue is due largely, 
if not wholly, to the inclusion of a greater number of nudibranchs. It is not unlikely that 
sufficient attention to our local nudibranchs on the part of a specialist would result in 
adding considerably to the number of species recorded for the region. As regards the 
shell-bearing species, however, we believe our list to be relatively complete for local 
waters. 
The average number of species of gastropods taken per dredge haul for the 458 
regular stations of the Survey was 6.8. This figure is only exceeded by that for the 
Pelecypoda. 
Those species which were so common as to be recorded from one-fourth or more of 
our dredging stations are listed herewith in the order of frequency: 
Number of stations. 
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4 Its apparent scarcity, even upon the stony bottoms off the shores of Cuttyhunk and Gay Head, renders the alternative 
explanation more likely{ or these points. 
