348 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
(or from low-water mark in some cases) to 65 or 100 fathoms or more, 
according to the species, Aplidium pallidum ranging from low water 
to 471 fathoms. Leptoclinides faerliensis stands alone among all onr 
species in being confined to depths of 100 fathoms and over, two 
small specimens being from 15S2 fathoms. The species of the second 
or southern group are exclusively shallow-water forms, but one of 
them being recorded from a depth as great as 18 fathoms. 
Distribution according to Character of Bottom. 
A majority of the species are recorded from stations with widely 
different kinds of bottom. Didemnopsis tenerum and Lissoclinum 
aurcum j^refer mud, but a stony or shelly bottom is usually most 
favorable on account of aft'ording the best o])portunities for attach- 
ment. When they occur on sandy or muddy bottom, the smaller 
species generally grow on large simple or compound ascidians or other 
large attached animals, not directly on the bottom itself. The species 
which form large colonies, as those of the genus Amaroucium, can 
grow on soft or loose bottom where there is no large fixed object for 
attachment, being then anchored by a quantity of fine gravel, sand, or 
mud, some of it more or less comj)letely included in the basal part of 
the colony. The existence of tidal currents carrying an abundant 
supply of food in suspension is a factor of more importance to these 
animals than the character of the bottom, provided only that the 
possibility of attaching or anchoring itself is afforded to the colony. 
In situations so favored it is not uncommon to find three or four kinds 
of ascidians, simple and compound, attached to the same shell or other 
object, or growing upon each other. Water that is even slightly 
brackish is, however, very unfavorable to them, and if much fresh 
water is received in a harbor or small bay their growth there may be 
prevented entirely. BotnjUus schlosscri has the habit of growing on 
eel-grass (Zostera) and upon floating timber or the bottoms of boats, 
as well as on rocks. Amaroucium pellucidum develops into the typical 
sand-incrusted form on a sandy bottom ontv. Where it grows on 
rocks or piles of wharves, the consteUafum form develops. Besides 
the last-mentioned species, Perophora viridis, Didemnum lutarium, 
Tetradidemimm albidum, and Amaroucium glahrum grow abundantly 
on the piles of wharves in some places. 
