10 THREE CRUISES OF THE “BLAKE.” 
The bathymetrical range of the mollusks is also connected 
with a wide geographical extension. According to Mr. Dall, if 
we consider the species dredged from the Atlantie Ocean north 
of a line drawn from Hatteras to Madeira, by all expeditions up 
to 1883, at greater depths than one thousand fathoms, we find 
that more than forty-two per cent live in some locality in less 
than one hundred fathoms. 
These species of mollusks have apparently taken advantage 
of the uniform conditions of existence in deep water, and have 
extended their range far from their original littoral ábode. 
There is a tolerable number of species, evidently unchanged, 
which occur all the way from a few fathoms, on the Florida 
coast, to two thousand fathoms in the adjacent deeps. A better 
knowledge of the littoral fauna of the tropics would undoubt- 
edly increase this percentage. We also notice that the per- 
centage of the genera or families peculiar to the continental and 
abyssal regions is small. 
The sea-urchins and starfishes have their fullest develop- 
ment in the continental zone, and there we find already many of 
the genera and families which have given so characteristic an 
aspect to the fauna of deep waters. Beyond that region live 
the eminently deep-sea types of the Pourtalesiw and Ananchy- 
tide, associated with a few starfishes and the strange order of 
holothurians, the Elasipoda. The ophiurans appear, of all the 
echinoderms, to flourish best in the deepest waters from which 
members of the class have as yet been dredged. The bathy- 
metrical range of many of the sea-urchins and ophiurans is very 
great, and extremes of depth extending to two thousand fath- 
oms or more are not uncommon. 
The stalked crinoids, as has been shown by Carpenter, are not 
strictly abyssal types ; on the contrary, seventy-five per cent of 
them have been brought up from depths of less than five hun- 
dred fathoms, — somewhat deeper than the limit of the conti- 
nental zone. As stated by Carpenter, out of the thirty-two 
recent species of stalked crinoids, nine species may be called 
littoral, living as they do at depths of less than one hundred 
fathoms. 
Comatule were dredged at fifty-seven out of the two hundred 
