since I feel that one already conversant with entomological names will find no difficulty in extending the already long list of parallel species, while to the general reader, who desires only the deductions of science, without entering upon the tedious processes by which they are obtained, a catalogue of mere technicalities, which fail to convey a single idea to his mind, will be equally useless and uninteresting.

I purposed in the present essay to trace, as far as possible, the mechanism of the agency by which the present distribution of species has been effected, and to reduce its most obvious results to some fixed principles. Fearful, however, lest my views should be considered as derived exclusively from a consideration of insects, and their phenomena of distribution, I prefer waiting until a sufficient familiarity with other sciences will enable me to be less partial in my choice of illustrations. I do this with the less regret as I find some of my deductions are at variance with many of the most ancient, and most firmly established prejudices of our nature, and before venturing any assertion, which even in appearance deviates from "general impressions," it is at least prudent to be supported by facts drawn from more extended observation than is furnished by one or two limited departments of knowledge.

In the rapids at Niagara have been observed large numbers of the singular animal described by Dekay (in the Zoology of New York) as a new genus of Crustacea, under the name of Fluvicola Herricki. They were attached to stones just below the surface of the water, and crawled but slowly; when seized, they endeavor to contract themselves into a ball.

These animals have a marvellous resemblance to the extinct group of Trilobites, although, as will be seen in the sequel, they are the larvæ of an insect. Mr. Agassiz informs me that a similar form has long been known to the zoologists of Continental Europe as Scutellaria amerlandica, but I have not been able to find any published account of it.

On turning over some stones near the river bank, I was agreeably surprised to find many specimens which had left the water for the purpose of changing into pupa. The elliptical shield of the superior

surface, which gives the animal its Crustacean appearance, was firmly adherent to the stone by its ciliated margin, and formed an excellent protection under which the later transformations could take place with safety. In fact, the superior shield being cast off with the larva skin, served in place of the cocoon or nest constructed by many larvæ, before transforming.

I regret that in the short account given by me at the recent meeting of naturalists in Cambridge, I was induced to speak of this discovery, without having access at the time to specimens. Those which I expected to find at Boston had been lost, and my former examination of the pupa collected by myself was very slight. I referred the insect to the order of Neuroptera, and I must here return my sincere thanks to my friend Dr. Harris, for a hint towards its true nature.

For the opportunity of examining some very large and well developed larvæ, I am indebted to my friend I. C. Brevoort, who procured them at Niagara in July of the previous year.

The body proper of the larvæ is elongate, the head being free, (i. e. not retractile,) but concealed under the large shield, like a prolongation of the dorsal epidermis of the prothorax. On each side are six small, approximate ocelli, anterior to which is the antenna, a little longer than the head, and two-jointed; each joint having a tendency to become divided at its middle, so that on a superficial inspection there would appear to be four joints. These organs are inserted at the outer extremity of the clypeo-cranial suture; the labrum is large, and a little emarginate in the middle. The lower part of the head is covered by a large mentum, which prevents the mandibles and maxillæ from being seen. The maxillary palpi are half the length of the antennæ, filiform, rather stout, and three-articulated, the joints being equal. The labial palpi are bent down and covered by the epidermis. In the very young larvæ the palpi are still shorter in proportion to the antennæ. A more full description of the parts of the mouth must be reserved for a separate treatise, when their structure can be illustrated by plates. The abdomen is furnished on each side with six bunches of long branchial filaments, which proceed from the interstices between the articulations; there is a larger bunch of filaments connected with the anal aperture, which

may be retracted, and is ordinarily not visible in dead specimens ; exterior to these filaments on each articulation is a small fovea. The articulation itself is prolonged each side, for a short distance between the lamina of the expanded epidermis, so that the outline of the proper fleshy portion is serrate. The legs are slender, the tarsus inarticulate, and furnished with a single claw.

The pupa is broadly oval, and depressed. The head is concealed under a hood formed by the prolongation of the epidermis of the prothorax. This hood is produced at the posterior angles, so that it becomes exactly similar to the thorax of a Lampyris.

The front between the antennæ is transversely elevated, so that the mouth is situated on its inferior surface. The antennæ are three times longer than the head, and inside of the pupa skin (in much developed specimens) are seen to be serrate, and eleven-jointed; the palpi are two-thirds the length of the antennæ, and are somewhat dilated at the extremity. The labial palpi are very short. The labrum is transversely cordate. The wings are bent under the body. The superior ones exhibit the structure of elytra, and have four slight longitudinal ribs the inferior are membranous, and show a slight transverse nervure near the middle. The abdomen is six-jointed and serrate at the sides, owing to the angular prolongation of each joint, and is entirely free from branchial appendages. The last joint is rounded. The feet are slender, and not armed with a claw. The mesopectus is deeply channeled.

After the description just given of the pupa, no one will doubt that the insect belongs to the Coleoptera; and from the serrate outline of the abdomen, one would be inclined to refer it to the groups possessing larvæ like the Lampyris, Lycus, &c. The separation of the prothorax and its great development, as well as the structure of the superior wings, absolutely exclude it from the Neuroptera, to which I at first referred it.

The peculiar structure of the head of the pupa, and the great length of the palpi, point clearly to Eurypalpus, a curious genus, which is placed by authors near Cyphon, which, as is well known, is closely allied to the Lampyrida. Eurypalpus differs very much from all the allied genera, in being aquatic. It is furnished with slender legs, but the tarsi are long, especially the last joint, which has two

very strong claws, (as in Macronychus) to fit it for clinging to stones in a rapid current. The mesopectus of Eurypalpus is likewise deeply channeled. The elytra are also furnished with three or four very obtuse elevated lines. As yet there is but a single species of the genus known. E. Lecontei, (Dej. Cat.) I am not aware that any description has been published of it.

Thus is settled the history of the transformations of an anomalous form, which has much perplexed naturalists for many years. Its history shows the care with which our investigation should be made, when we are upon unknown ground. But where the homologies of the animal with other aquatic larvæ provided with branchia are so exact, it is a little remarkable that its larval character should remain. so long unnoticed. The only difference between it and an ordinary larva (either of Coleoptera or Neuroptera) is the prolongation of the dorsal epidermis, to form a shield under which the true body is concealed. Similar prolongations are found in nearly all orders of insects.

I know not how Dr. Dekay fell into the mistake of considering the elongate palpi as a second pair of antennæ: and surely such an anomalous form as a Crustacean with six legs, and a head separate from the thorax, deserved a more careful examination, before receiving a definite place in the system.

The figure in the New York Zoology, (as the animal is very peculiar in its form,) bears a certain vague resemblance to what it was intended to represent; but for all systematic purposes, it is, like nearly all the plates in that part of the State Survey, perfectly worthless.

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striatella, Anthony. Fort William, Cape Gourganne, N. E. of St. Ignace. concava, Say. Niagara.

arborea, Say. Mackinaw, Fort William, Cape Gourganne, St. Ignace. electrina, Gould. Cape Gourganne.

chersina, Say. Michipicotin, Cape Gourganne.

1 Vitrina limpida, Gould. Cape Gourganne.

Succinea ovalis, Gould. Fort William.

avara, Say. Niagara.

Physa heterostropha, Say. Black River, Pie Island, Fort William.

1 VITRINA LIMPIDA, Gould (V. pellucida, Say, in Long's Expedition. II. 258.) Having made a critical comparison of our Vitrina with the V. pellucida of Europe, with which species it has hitherto been regarded as identical, I am induced to believe that they are different species. The American shell is more globose; the plane of the aperture is more oblique, and the basal portion of the lip sweeps round from the columella in a rapidly curving arc, instead of stretching off almost horizontally; indeed the whole aperture is more nearly circular. These differences become quite obvious when the shell is greatly magnified. The color of the European shell is always more or less green or yellow, whereas the American specimens are colorless, and decidedly more fragile. In size, they are about one fourth smaller than the foreign ones, and have, at least, half a whorl less in the spire. It is indeed more nearly like V. subglobosa, Mich. which, however, has a much more elevated spire, and its basal face much more inflated.