BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 145 
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 Twrbonilla 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 Trttva trivitiata, Anachis avara, Ilyanassa 
obsoleta, Polynices heros, P. triservata, 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.¢ 
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 Chetopleura apicu- 
lata, which was seldom taken except alive, was uot 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. 
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 1—13 0 
