BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 1 45 



mens from one of the dredge hauls, believing these all to be of one species and therefore 

 regarding them as representative of the specimens taken from various other dredge 

 hauls. Since an examination by Dr. Bartsch revealed the presence at times of three 

 or four species of Turbonilla from a single dredging station, it is obvious that such rec- 

 ords as are not based directly upon preserved material taken at one station are worth- 

 less so far as specific names go. It has been necessary, therefore, to record a large 

 proportion of our Turbonillas merely as "Turbonilla sp. ;" and thus our data for this 

 interesting genus are in a large degree rendered valueless. 



There are some other possible sources of error in interpreting our records which 

 have no relation to defects of method. For example, for certain of the gastropods the 

 apparent distribution is doubtless much more extensive than the actual one, owing to 

 the transportation of their shells by hermit crabs. This is notably true of the intro- 

 duced periwinkle, Littorina litorea, which is typically and indeed almost exclusively a 

 littoral (intertidal) species. Nevertheless the shells of this mollusk were found at 131 

 stations, occurring even at depths of 10 or 15 fathoms. Other gastropods whose shells 

 are most commonly occupied by the paguri are Tritia trivittata, Anachis avara, Ilyanassa 

 obsolete,, Polynices heros, P. triseriata, P. duplicata, Busycon canaliculatum, B. carica, 

 Urosalpinx cinereus, and Eupleura caudata. To what extent the distribution of these 

 species, as plotted in the charts, has been the result of transportation by hermit crabs 

 is impossible to state. It is not recorded in all instances whether or not a given shell 

 was inhabited by one of the crabs, and in any case the presence of the latter in a shell 

 would not by any means prove that this had been carried to any great distance beyond 

 the point where the mollusk lived. 



In the case of certain thin shells of light weight it is quite probable that the tidal 

 currents have often been instrumental in carrying them beyond the original habitat of 

 the animal, though we can not, of course, assume this in any single case. Man, like- 

 wise, has almost certainly been responsible for the occurrence of the shells of one species, 

 at least, in unexpected localities. The large oyster shells which have been taken not 

 infrequently in various parts of the main channel of Vineyard Sound have probably 

 been cast overboard from passing vessels, since living oysters of our American species 

 are not known to occur in such situations. 



In the charts for the Mollusca, as for other shell-bearing organisms, we have indi- 

 cated the known presence of living specimens at a given station by means of a circle 

 surrounding the star. It must not be inferred, however, that only dead specimens 

 were taken, at the other stations. Absence of the circle denotes either that the occur- 

 rence of shells only was specified or merely that living specimens were not recorded.* 1 

 It is quite certain that living mollusks were of much more frequent occurrence in our 

 dredge hauls than the circles upon the distribution charts would imply. This is prob- 

 ably particularly true of the small gastropods. Indeed, the chiton Chcetopleura apicu- 

 lata, which was seldom taken except alive, was not commonly designated as living or 

 dead in the dredging records. For this reason, it has been necessary to omit the circles 

 from the chart. 



For the remainder of this discussion it will be best to consider the classes of Mollusca 

 separately. 



For certain mollusks we have employed the circle whenever the nature of the record rendered it probable that living speci- 

 mens were taken, even though this was not expressly stated. For example, the note "on [or in] hermit crab shells," when 

 applied to Crepidula, has been regarded as equivalent to a record of living specimens. 

 16269° — Bull. 31, pt I — 13 IO 



