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 -g-inch. Exeter College, Oxford, Feb. 18, 1878. XXIX. — Studies on the Hydroida. By C. Mereschkowsky. [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 Hydromedusfe,
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