8 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [302] 
water and high water, nor the relative duration of the ebb and flow are 
coincident, very powerful currents set through the passages, between 
the Elizabeth Islands, connecting these two bodies of water. This is 
most noticeable in the case of Wood’s Hole, because there the channel 
is narrow and shallow, and much obstructed by rocks. These channels 
are, therefore, excellent collecting grounds for obtaining such animals 
as prefer rocky bottoms and rapidly flowing waters. 
The shores of Vineyard Sound and Buzzard’s Bay are quite diversified 
and present nearly all kind of stations usually found in corresponding 
latitudes elsewhere, except that ledges of solid rock are of rare oceur- 
rence, but there are numerous prominent points where the shore con- 
sists of large rocks or boulders, which have been left by the denuda- 
tion of deposits of glacial drift, forming the cliffs along the shores. 
Sandy beaches are frequent, and gravelly and stony ones occasionally 
occur. Muddy shores are less common and usually of no great extent. 
In Buzzard’s Bay the bottom is generally muddy, except in very 
shallow water about some of the islands, where patches of rocky bot- 
tom occur, and opposite some of the sandy beaches where it is sandy 
over considerable areas. Tracts of harder bottom, of mud or sand, 
overgrown with algwe, occasionally occur. In Vineyard Sound the bot- 
tom is more varied. It is sandy over large districts, especially where 
the shoals occur, and in such places there are but few living animals, 
though the sand is often filled with dead and broken shells, but in 
other localities the sand is more compact and is inhabited by a peculiar 
set of animals. Other extensive areas have a bottom of gravel and 
small stones and broken shells; on such bottoms animal life is abun- 
dant, and the entire bottom seems to be covered in some places by sev- 
eral kinds of compound ascidians, which form large masses of various 
shapes, often as large as a man’s head. In still other places, chiefly off 
rocky points and in the channels between the islands, rocky bottoms 
occur, but they are usually of small extent. Muddy bottoms are only 
occasionally met with. They occur in most of the deep areas which are 
isolated, and sometimes in the deep channels, but are more common in 
Sheltered harbors and coves. 
In Nantucket Sound and Muskeget Channel the bottom is almost 
everywhere composed of sand, and the same is true of an extensive 
area to the east and northeast of Nantucket Island, where shoals of 
moving sand are numerous and often of large size, but in the partially 
sheltered area on the north side of Nantucket, there is more or less mud 
mixed with the sand. 
For greater convenience the following subdivisions have been adopted 
in describing the animals of the bays and sounds: 
1. Rocky shores, between high-water and low-water marks. 
2. Sandy and gravelly shores. 
3. Muddy shores and flats. 
4. Piles of wharves, buoys, &e. 
