BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. I5I 



as stones and shells." On the other hand, it is not improbable that the temperature 

 factor has been partly responsible for the distribution of ChcBtopleura 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, Clustopleura 

 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. 



HI. 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 1 7 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. 



Tritia trivittata 373 



Crepidula fomicata 326 



Anachis avara 295 



Crepidula plana 291 



Astyris lunata 245 



Polynices heros 165 



Urosalpinx cinereus 156 



Polynices triseriata 144 



Littorina litorea (shells only) 131 



o Its apparent scarcity, even upon the stony bottoms off the shores of Cuttyhunk and Gay Head, renders the alternative 

 explanation more likely f or these points. 



