BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 1 53 



to the inshore stations. The ease of Crepidula convexa is peculiar, inasmuch as the two 

 other local species of Crepidula are both of very general distribution. The distribution 

 of this species is particularly unintelligible, in view of the fact that none of our hermit 

 crabs, upon whose shells it finds lodgment, are in any degree restricted to the shallower 

 waters along shore. Yet this mollusk was recorded from 45 of the Phalarope and Blue 

 Wing stations, as compared with only 16 Fish Hawk stations; and of these last, indeed, 

 there is reason for regarding a considerable number as doubtful. This species is known 

 to be the commonest Crepidula upon the smaller hermit crabs in shallower waters 

 near shore, but it is difficult to understand why this mollusk is not more frequently 

 carried by its hosts into the deeper waters as well. 



As in the case of the Pelecypoda, certain species of gastropods are restricted to the 

 colder waters of the Sound. The only two to be mentioned are Buccinum undatum and 

 Crucibuhim striatum. The former was likewise taken at 6 of the 7 regular stations at 

 Crab Ledge, and is known to be a predominantly northern species. Such is not true 

 of Crucibulum, however, and we are at a loss to explain this peculiarity in its local 

 distribution. 



Both of these species (and these alone among the gastropods) were taken pre- 

 dominantly at depths of 10 fathoms or more. In fact Crucibulum was dredged only once 

 by the Phalarope, and was never taken in less than 10 fathoms of water. 



Certain species among those charted are seen to be less common, or to be wanting 

 altogether, in the western half of the Sound, although present in the eastern half. Such 

 are Cerithiopsis cmersonii and Vermicularia spirata. Two others (Eulima conoidea and 

 Seila terebralis) might also be mentioned here, though neither has been taken with 

 sufficient frequency to warrant our drawing any general conclusions. 



The distributions of two species of gastropods as portrayed upon our charts are 

 obviously largely fictitious. We refer to Littorina litorea and Ilyanassa obsoleta, both of 

 which are known to be restricted, when living, to the immediate vicinity of the shore. 

 The broadcast way in which the shells of these species, particularly the former, are 

 strewn around the local sea floor testifies strongly to the part played by hermit crabs in 

 transporting them. 



Several genera comprise species which display among themselves interesting differ- 

 ences of habitat. Such are Busycon, Crepidula, Littorina, and Polynices. For most of 

 these the differences may readily be seen by reference to the charts. The case of Crepi- 

 dula has just been discussed; that of Polynices receives some mention in chapter v 

 (p. 186). As regards Littorina, only one species is represented upon our chart, and this 

 latter in no way represents the distribution of the living animals. In the catalogue of 

 species (section in), however, the differences in their respective habitats have been 

 briefly indicated. 



A glance at the subjoined lists shows that our local assemblage of gastropods, or at 

 least the commonest and most representative among them, are even more dominantly 

 southern than are the pelecypods. Of the 27 species there considered, 22 are to be 

 regarded as southern, 3 as northern, while the remaining 2 are not to be assigned to 

 either category. 



Predominantly northern (3). 



Buccinum undatum (chart 166). . . .Arctic Sea to Charleston Harbor. 



Polynices heros (chart 187) Labrador to Virginia. 



Polynices triseriata (chart 188) Labrador to off Hatteras. 



