M. C. Mereschkowsky on the Hydroida. 239 
beneath very pale, almost white. Five dark violet stripes, 
a mesial and two pairs of lateral, extending along the entire 
length of the dorsal surface. The mesial stripe narrow and 
linear, the succeeding pair broad and band-like, and the 
outermost pair again linear. The outermost pair placed at a 
short distance from the lateral margin of the upper surface, 
and the band-like pair at half the distance between these and 
the central stripe. Just behind the head the two lateral 
bands on either side fuse together, and form a pair of broad 
dark patches. 
Faint and narrow violet stripes mark the margin of the 
ambulacral line on the under surface of the body. 
Length of the single specimen 9 inches; extreme breadth 
of the body + inch, of the head + inch. 
Exeter College, Oxford, 
Feb. 18, 1878. 

XXIX.—Studies on the Hydroida. By C. Merescukowsky. 
[Plates XIII., XIV. & XV.] 
I. Morphological Considerations. 
THE human mind has not the power of retaining in its 
memory the representations of all the concrete objects which 
are presented to its five senses ; for the number of these objects 
and of facts is too immense for its faculties, which are still so 
imperfectly developed. But, at the same time, the mind de- 
sires to be in possession of as many facts as possible; hence 
the tendency to generalization and the double character of 
every science: on the one hand, we have concrete facts 
without any bond between them, without any idea, serving 
only as raw material; on the other, generalizations, more or 
less abstract ideas. Not only every science, but even every 
branch of each science, every group of events or facts, may 
therefore have its philosophy—that is to say, its generaliza- 
tions, its ideas, its laws which govern the facts. 
The usefulness of these laws or generalizations, even in the 
case of small groups of events, cannot be doubted; in reality 
it is often only by taking advantage of them that a thinker can 
arrive at generalizations of a higher degree, without the neces- 
sity of busying himself in the midst of thousands of little facts 
and minute details. 
In the following pages I shall speak of a group of 
facts which may be observed among the Hydromeduse, 
