Mr. GC. Collingwood on Oceanic Forms of Hydrozoa. 313 
elegant, brown, turquoise-studded species, similar to one I had 
already obtained in Victoria Harbour, Labuan, and of which, it 
may be here mentioned, I found a small crab within the um- 
brella, beneath which it appeared to reside. 
To show the vast numbers of these animals which swim freely 
in the ocean, I will mention that, in the Atlantic, in lat. 34° S. 
and long. 17° W., we encountered a shoal of these animals, all 
of the same species, the individuals of which were among the 
most beautiful in form and colouring that I have ever met with. 
Just before sunset we passed through them for a space of two 
hours, during which time we had traversed ten miles. Sup- 
posing that this shoal were at least as broad as long, it was easy 
to calculate roughly that there could not be less than thirty 
millions of individuals constituting it, an estimate probably far 
below the truth. I procured one, and made a careful drawing 
of it while still living. 
The only exception I met with to the rule I have mentioned 
(namely, that when Hydrozoa floated they appeared in consider- 
able shoals of one species only) occurred in the great calms 
which I encountered in the North Atlantic Ocean, in the first 
fortnight of July, and which extended more or less over upwards 
of a thousand miles, during which, on two or three occasions, I 
saw several species of Hydrozoa mingled with vast numbers of 
compound Ascidians. Some of them were new and strange 
forms, such as I have nowhere seen figured, some abundant, 
others but few in number, only appearing occasionally, and 
therefore very difficult to capture from a moving ship. One of 
these I did succeed in taking; but there were at least three or 
four species besides the Physalie and Velelle. 
It occasionally happened that the observation of a shoal of 
Hydrozoa pointed out some curious facts from which interesting 
deductions might be made. Thus, while passing through the 
Indian Ocean, in lat. 13° N., during an entire day (March 17), 
we ran through shoals of Aurelia, meeting from time to time 
patches in which they were too numerous to be counted, and in 
each of which there were many hundreds. A noticeable fact I 
remarked with regard to them, viz. that, without any exception 
.that I could discover, these Aurelie were, during the whole day, 
swimming in the same direction, or with the wind. We were 
steaming nearly due east, and a breeze was blowing a little 
south of east ; and the umbrellas were all inclined one way, and 
pointing in the direction towards which the wind was blowing. 
On another occasion, in a dead calm, on a beautiful day, off 
the river Min, I observed great numbers of a large white species. 
The edges of the umbrella were frilled, and numerous long and 
delicate threads stretched out straight and parallel; but what 
