180 BULLETIN OF THE BUREAU OF FISHERIES. 
ensis, Cancer borealis, Ovalipes ocellatus, Pagurus acadianus, Astarte castanea, Astarte 
undata, Venericardia borealis, and Amaroucium stellatum. Now, a number of the fore- 
going species, and in general a considerable proportion of those species which are 
limited to the deeper waters, have already been mentioned among the northern forms 
whose distribution is determined locally by temperature conditions. It must be 
repeated, however (see p. 28), that the waters of the western end of Vineyard Sound 
are little if any deeper on the average than those in the vicinity of Nobska and West 
Chop. The preference of these species for deeper waters is shown by their scarcity in 
the adlittoral zone. Certain of them, indeed, were dredged only by the Fish Hawk. 
It is more than likely that the somewhat lower summer temperature of these bottom 
waters, as compared with those skirting the shore, is the factor responsible for the 
restriction of some species to the former. ‘The temperature factor is not the one directly 
concerned, however, in the case of all of the animals named. The distribution of 
Ovalipes, for example, is probably wholly determined by the character of the bottom. 
It is indeed known to occur on sand flats in shallow, warm water. The case of Amar- 
oucium stellatum is interesting, since, although a deep-water species in the sense here 
employed, it is for the most part restricted to the more easterly portions of the Sound, 
where the bottoms are gravelly or stony. Thus its preference for deeper waters does 
not appear to be related to the temperature factor, though this is not entirely certain, 
since the deeper waters are everywhere somewhat cooler in summer than are the shoaler 
ones. The marked restriction of this species to the former is in striking contrast to the 
condition shown by the related Amaroucium pellucidum constellatum (=A. constellatum 
Verrill), which, although associated with A. siellatum at various points, is likewise 
found in profusion in shallow waters and even upon piles. 
The vertical distribution of marine organisms is commonly designated by the term 
‘bathymetric,’ and it has been sometimes supposed that depth was one of the primary 
factors determining distribution. There are, of course, at least four factors bound up 
in this one, viz, pressure, temperature, light, and gas content. Now, it is not at all 
certain to what degree, if any, pressure influences distribution. For the limited depths 
within our region, we may certainly leave it out of account. 
Temperature is, as we have seen, definitely correlated with depth in the sea, just 
as it is with altitude on land. But there is, in local waters, little difference between 
surface and bottom temperature, except in those portions of the Sound and the Bay 
which adjoin the open ocean. Some of the cold-water species inhabiting these last 
are, as just stated, restricted to the greater depths. On the other hand, the restriction 
of certain species (see above) to the shallow water immediately skirting the shore may 
be due in some cases to the palpably higher temperature commonly met with at such 
points during the summer. 
The relation of light to depth has been treated at some length in the botanical 
section of this report (p. 447-449), to which the reader is referred. It is likely that 
for relatively slight depths, such as those we are considering, the light factor has little 
direct effect upon the bathymetric distribution of animals. Indirectly it may be of 
influence in the case of certain forms which dwell upon alge, and it is possible that some 
of the adlittoral species which have been discussed above are limited in this way. 
It may be repeated in conclusion, however, that, as regards the species taken 
during our dredging operations, the great majority show little or no evidence of 
bathymetric distribution. 
