we introduce another principle, by which we can weigh the real value of those remarkable differences. Such a principle, I think, we have in the metamorphosis of embryonic life. Indeed, if it can be shown, that besides the differences which exist in all fishes between their earliest forms and their full grown state, there are peculiarities in sharks, skates, and lamprey eels, common to all of them, from an early period of development, which remain characteristic throughout life, it must be acknowledged that these families belong to one and the same great group, notwithstanding their extreme differences in their full-grown condition. Now, such facts exist. In the first place, it is impossible, without disturbing their true affinities, to consider an extraordinary development of pectoral and ventral fins as a standard to appreciate fundamental relations between fishes, as in all fishes, without exception, they are both wanting in earlier life, and as there is scarcely a family in which ventrals at least, are not wanting in some genus or other. We might just as well place Petromyzons among the eels, as their common English name purports, on the ground of the deficiency of their abdominal and thoracic organs of locomotion, as separate them from the other Placoids. Again, the peculiarities in the development of the dorsal, caudal, and anal fins in sharks and skates, and the difference which exists between them and the Petromyzons, indicate in no way their affinity or their difference; in Petromyzon we have the embryonic condition of vertical fins, where a continuous fold in the skin of the middle line extends, as in all embryo fishes, from the back round the tail, towards the abdominal region. In the sharks we have distinct vertical fins, as they generally grow out of the continuous embryonic odd fin; whilst in the skates these fins disappear almost entirely, or are considerably reduced. That animals in their embryonic condition are neither so elongated as many of cylindrical form in their full-grown state, nor so short as some others, is ascertained by the embryology of snakes and toads. Thus all the great external differences which exist between skates and sharks on one side, and Petromyzon on the other, do not shew that these animals do not belong to the same na

tural group, as we have even among the full-grown ones, what we may call transitions between the extreme forms; for instance, sharks with more elongated body than others, with more extensive vertical fins, even with two dorsals and some without ventrals. Again, the remarkable form of skates arises solely from an extraordinary development of the pectorals; they are nevertheless closely allied to sharks, notwithstanding the striking difference in the position of the gill opening.

As for the anatomical differences which exist among these fishes, and upon which so much stress is placed as to make the want of a heart, in Amphioxus, the foundation for a peculiar class to include that single fish, let us not forget that there is an epoch in embryonic life, when no vertebrated animal has yet a heart; when the vertebral column is a mere soft continuous cord; when the brain is scarcely subdivided into lobes; when the head, as such, is not yet distinct from the trunk; when the mouth is a mere circular opening at the anterior extremity of the body; when the gills are simple fissures on the sides of the head, or what is to be a head, without branchiostegal rays, or operculum, or protecting covering of any kind.

Whoever is familiar with the anatomy of fishes must perceive, after these remarks, that the peculiarities which characterise Petromyzon have a bearing upon the embryonic condition of their structure, even in their full-grown state, and do not, by any means, mark a difference between them and the sharks and skates, any more than between them and any other family of fishes. On the contrary, should it be possible, after these statements, to shew that there are important characters, common to Petromyzon, sharks, and skates, notwithstanding their extreme external differences, it should be acknowledged that Cyclostomata and Plagiostomata are only different degrees of one and the same great type. Now, such characters we have; in the first place, in the structure of the mouth, which differs so widely from that of the other fishes, and agrees so closely in all Placoids, as Müller himself has shewn in his Anatomy of Myxinoids. Next, the teeth also agree, in being arranged in

several concentric series, and also in their microscopical structure, as well as in their mode of attachment to the skin lining the jaw, and not to the bone itself. We have other hints of the relation between Cyclostomes and Plagiostomes in their spiracles, and also in their numerous respiratory apertures, so that, after due consideration, I come to the conclusion that the Myxinoids and Petromyzons, far from being the types of peculiar sub-classes, are simply embryonic forms of the great type to which sharks and skates belong, bearing to these powerful animals, in a physiological point of view, the same relation which exists between Ichthyodes and the tailless Batrachians.

Of Cyclostomata, two species have been mentioned as occurring in the colder parts of North America, both referred by Dr Richardson to the genus Petromyzon proper, but of which I have seen no trace myself in the great lake region, though I know Petromyzons to occur below Niagara Falls. However, I am able to add a new species of this family to the fauna of those waters, which belongs to the genus Ammocates, and was found in the mud in Michipicoton River, at the landing-place of the factory, the first specimens of which were picked up by the students, when dragging their canoes along the shore.-Agassiz on Lake Superior, p. 249.

On Fossil Rain Drops.*

Mr Desor communicated some observations made by Mr Whitney and himself in reference to the probable origin of the so-called fossil rain drops, which, in this country, we found on slabs of red sandstone, as well as Potsdown sandstone.

He said it had already been noticed by Mr Teschemacher that these so-called rain drops, when closely examined, are found to differ in several respects from the impressions made by the rain on a beach, where each drop produces an impres

* Proc. Boston Soc. Nat. Hist., 1850.

sion surrounded by a rough crest, more or less elevated according to the force of the rain. The fossil impressions on sandstone, on the contrary, are generally flat and smooth. Besides, there is hardly a shower in which the rain drops are not numerous enough to cover the whole or nearly the whole ground, whereas the fossil impressions are generally scattered, and so few in number, that it seems almost impossible to ascribe them to rain.

Mr Desor said, that whilst encamped on the border of Lake Superior, they had several opportunities of studying the action of the waves on the beach during a heavy surf, when they are driven beyond their usual range. It was noticed that when the waves retired from the higher part of the beach, where the slope was less steep, there could be seen several kinds of impressions in the act of forming, some large and flat, others small and deep (like those which on the sea shore are generally ascribed to worms or shrimps), and others likewise deep, but surrounded by a sort of annular, smooth rim. These different kinds of impressions are all produced by the same cause, operating in the same way, namely, air-bubbles, which are formed in the waves of the surf, when rolling over the beach. If an air-bubble becomes buried in the sand, so that, in order to escape, it has to make its way through the new formed stratum of sand, it forms a deep and narrow hole. If the air, instead of escaping at once, bubbles up several times, then it raises around the hole a small and smooth rim, which may be compared to a miniature crater of a volcano. If, on the contrary, the air-bubble remains at the surface and bursts, then it causes a flat and rather large impression. According to Messrs Whitney and Desor, these different forms of impression, arising from air-bubbles, are sufficient to account for most impressions which have hitherto been considered as the effect of rain. Such impressions of airbubbles are most perfect where the slope of the beach is very gentle. Where the slope is more or less steep, the sand becomes too much hardened, under the pressure of the waves, to allow these delicate impressions to be produced.

A sketch was exhibited, showing those different forms of

impressions, and their striking contrast with impressions of rain drops from the same beach, mouth of Carp River, Lake Superior.

Mr Teschemacher said, that he had seen fossil rain drops, so called, with an elevated ridge crossing them; an appearance easily explained by Mr Desor's hypothesis, but incompatible with the supposition that they were caused by rain.

Professor Agassiz said, that on the mud-flats at Cambridge, he had noticed impressions made in the way described by Mr Desor at Lake Superior.-American Journal of Science and Arts, vol. x., 2d Series, No. 28, p. 135.*

On the Fossil Crocodilia of England.

On reviewing the information which we have derived from the study of the fossil remains of the procœlian Crocodilia, that have been discovered in the Eocene deposits of England, the great degree of climatal and geographical change, which this part of Europe must have undergone since the period when every known generic form of that group of reptiles flourished here, must be forcibly impressed upon the mind.

At the present day the conditions of earth, air, water, and warmth, which are indispensable to the existence and propagation of these most gigantic of living Saurians, concur only in the tropical or warmer temperate latitudes of the globe. Crocodiles, Gavials, and Alligators now require, in order to put forth in full vigour the powers of their cold-blooded constitution, the stimulus of a large amount of solar heat, with ample verge of watery space for the evolutions which they practice in the capture and disposal of their prey. Marshes with lakes, extensive estuaries, large rivers, such as the Gambia and Niger that traverse the pestilential tracts of Africa, of those that inundate the country through which

* Many of the so-called fossil foot-marks will, we believe, turn out to be imaginary.-Edit. Ed. Ph. Journal.