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adopted—a classification, in our opinion, of the highest physiological and geological value.
It is one of the essential characters of the integument of fishes to be protected by scales of a peculiar form and structure. Tbis external protection is in direct relation with the internal organization of the animal; and Dr. Agassiz found, upon a close esamination of the scales, that tishes might be separated into more natural orders than those already acknowledged. Proceeding upon this foundation, he has established four orders bearing some relation to those of Artedi and Cuvier. One of these orders, hitherto entirely misunderstood, is almost exclusively composed of genera whose species occur in the most ancient strata only ; and here we may observe upon the great advantage of this mode of classification as applied to fossils; for the enamelled scales—and this condition of the scale is more particularly observable in those fishes which existed at the earlier geological epochs—are much less liable to decomposition than the osseous parts; and we are not without instances where the figure or case of the fish has been entirely preserved, while no trace of the bones is to be found.
But to return to the system of Dr. Agassiz. His four orders are
First, the Placoidians (Thag, a broad plate). These are characterized by a skin covered irregularly with enamelled plates, often of large size, but sometimes only developed in the form of small points, like the shagreen on the skin of many sharks, and the thorny tubercles on the integument of rays. This order comprehends all the cartilaginous tishes of Cuvier, with the exception of the sturgeons.
The second order consists of the Ganoidians (yavos, splendour, from the brilliant surface of their enamel). These are characterized by angular scales, formed of horny or bony plates, protected by a thick layer of enamel. The Ganoidians form upwards of sixty genera, of which fifty are extinct.
The third order includes the Ctenoidians (xTEIS, a comb). The scales of these are pectinated on their posterior margin, like the teeth of a comb. They are composed of laminæ of horn or bove, but have no enamel. The scale of a perch affords a familiar example of this construction; and the fishes of this order are the Acanthopterygians of Artedi and Cuvier, with the exception of those which have smooth scales, and with the addition of the Pleuronectes or flat-fishes, as they are vernacularly called.
The Cyclöidians (xuxãos, a circle) form the fourth order. The general character of the scales of the families of this order is smoothness and a simple margin; though they are frequently ornamented or sculptured, as it were, with various figures or patterns
on their upper surface. Laminæ of horn or bone without any enamel are the ingredients in their composition. The Cyclöidians of Agassiz are principally Malacopterygians, or soft-finned fishes, comprehending, however, in addition, all those families excluded from the Acanthopterygians of Cuvier, while the Pleuronectes, which are removed to the Ctenoidians, must be deducted.
The test of the scale is so sure that a single one will often deternine the genus, and even the species, with as much certainty as the shell of a mollusc would determine the genus or species of its inhabitant; and one important geological result has already attended the researches of our ichthyologist, for the age and place of several formations hitherto unexplained, as well as the identity of others, have been clearly ascertained by a knowledge of the fossil fishes which they contain, in consequence of his acute and accurate labours. Here, then, we have a great addition to the dynamics of geology.
* It must be obvious,' says Dr. Buckland, that another and most important branch of natural history is enlisted in aid of geology, as soon as the study of the character of fossil fishes has been established on any footing which admits of such general application as the system now proposed. We introduce an additional element into geological calculations; we bring an engine of great power, hitherto unapplied, to bear on the field of our inquiry, and seem almost to add a new sense to our powers of geological perception.'
Some of the results already obtained, and thus eulogized, may be gathered from the following abstract of a paper by Professor Agassiz, read before the Geological Society of London on the 5th November, 1834.
• If we estimate the number of fishes now known to amount to about eight thousand species, we may state that more than threefourths of this number belong to two only of the above-mentioned orders; namely, the Cycloidians and Ctenoidians, whose presence has not yet been discovered in the formations inferior to the chalk. The other fourth part of living species is referable to the orders Placoidians and Ganoidians, which are now far from numerous, but which existed during the whole period which elapsed since the earth began to be inhabited, to the time when the animals of the green sand lived.
It is to be observed that, in fishes, more considerable differences may be remarked within narrow geological limits than among inferior animals. We do not see in the class of Fishes the same genera, nor even the same families, pervading the whole series of formations, as takes place among zoophytes and testacea. On the contrary, from one formation to another, this class is represented by very different genera, referable to families which soon become extinct, as if the complicated structure of a superior organization could not be long perpetuated without important modifications; or rather,
as if animal life tended to a more rapid diversification in the superior orders of the animal kingdom, during equal periods of time, than in its lower grades. With respect to this, it is with fishes nearly as with mammifers and reptiles, whose species, for the most part but little extended, belong ai a short distance in the vertical series to different genera, without passing insensibly from one formation to another, as is generally admitted to be the case with certain shells. One of the most interesting facts which Mr. Agassiz has observed is, that he does not know a single species of fossil fish which is found successively in two formations, whilst he is acquainted with a great number which have a very considerable horizontal extent.
• The fish of the tertiary formations approach nearest to recent fish, yet hitherto the author has not found a single species which he considers perfectly identical with those of our seas, except the little fish which is found in Greenland in geodes of clay, and whose geological age is unknown to him.
The species of the crag of Norfolk, the superior subapennine formation, and the molasse, are related for the most part to genera now common in tropical seas; such are the Platax, the large Carcharias, the Myliobates, with large palatal plates, and others. In the inferior tertiary formations, the London clay, the calcaire grossier of Paris, and at Monte Bolca, a third at least of the species belong to genera which exist no longer. The chalk has more than two-thirds of its species referable to genera which have now entirely disappeared. In it we already see even some of those singular forms which prevail in the Jurassic series. But, as a whole, the fishes of the chalk recall more forcibly the general character of the tertiary fishes than that of the species of the Jurassic series.
• If we paid attention only to fossil fish in the grouping of geological formations on a large scale, the author thinks it would be more natural to associate the cretaceous with the tertiary strata, than to place the former among the secondary groups. Below the chalk there is not a single genus which contains recent species, and even those of the chalk which have them, contain a much greater proportion of species which are only known as fossil. The oolitic series, to the lias inclusive, forms a very natural and well-defined group, in which also must be included the Wealden, in which Mr. Agassiz states he has not found a single species referable even to the genera of the chalk. Henceforth, the two orders which prevail in the present creation are found no more; whilst those which are in a small minority in our days, appear suddenly in great numbers. Of the Ganoidians, those genera which have a symmetrical caudal fin are found here, and, among the Placoidians, those above all predominate which have their teeth furrowed on both the external and internal surface, and have large thorny rays. For it is now certain that those great rays which have been called Ichthyodorulites, belong neither to Silures nor Balistæ, but are the rays of the dorsal fin of the great Squaloids (true sharks), whose teeth are found in the same strata.
‘On leaving the lias to come to the inferior formations, we observe a great difference in the form of the posterior extremity of the body in the Ganoidians. All have their vertebral column prolonged at its extremity into a single lobe, which reaches to the end of the caudal fin, and this peculiarity extends even to the most ancient fishes. Another observation worthy of attention is, that we do not find fishes decidedly carnivorous before the carboniferous series ; that is to say, fish provided with large conical and pointed teeth. The other fish of the secondary series below the chalk appear to have been omnivorous, their teeth being either rounded, or in obtuse cones, or like a brush.
• The discovery of coprolites containing very perfect scales of fish which had been eaten, permits us to recognise the organized beings which formed the food of many ancient fish; even the intestines, and, in some fossil fish of the chalk, the whole stomach are preserved, with its different membranes. In a great number of fish from Sheppey, the chalk, and the oolite series, the capsule of the bulb of the eye is still uninjured ; and in many species from Monte Bolca, Solenhofen, and the lias, we see distinctly all the little blades which form the branchiæ.
• It is in the series of deposits below the lias that we begin to find the largest of those enormous sauroid fish whose osteology recalls, in many respects, the skeletons of saurians, both by the closer sutures of the bones of the skull, their large conical teeth, striated longitudinally, and the manner in which the spinous processes are articulated with the body of the vertebræ and the ribs at the extremity of the spinous processes.
• The small number of fish yet known in the transition formations does not as yet permit the author to assign to them a peculiar character ; nor has he discovered in the fossil fish of strata below the green sand any differences corresponding with those now observed between marine and fresh water fish, so that it is impossible, on ichthyological data, to decide on the freshwater or marine origin of the ancient groups.'
The general inference then appears to be that fossil fishes approach nearest to existing genera and species in the more recent tertiary deposits ; and that, in formations of the greatest antiquity the difference is greatest, while, in the intermediate strata, intermediate changes of ichthyological condition are obvious. Moreover it is evident that all the great alterations in the character of fossil fishes have occurred simultaneously with the most important changes in the other departments of fossil zoology, as well as in that of fossil botany; and that these revolutions were accompanied also by an alteration in the mineral condition of the deposits.
Thus, the genera that prevail in the carboniferous strata disappear after the deposition of the zechstein or magnesian limestone. After the zechstein was formed, those of the oolitic series were introduced—and suddenly ceased to exist upon the commencement of
the the chalk formation, in which we first see an approximation to existing genera. The lower tertiary strata of London, Paris, and Monte Bolca present forms still more similar to fishes now living ; while the fossils of Qeningen and Aix approach yet nearer to existing genera, though every one of the species seems to be extinct.
This summary is not interesting merely to the geologist,—the zoologist and physiologist will find in it ample materials for thinking, in relation to the creative influence and the development of animal life.
Some of our readers may be induced to smile at the gravity with which the coprolites (those fæcal balls first discovered by Dr. Buckland, and which have thrown so much light on the organization of fossil animals) are introduced in the abstract above quoted. In the Bulletin of the Imperial Society of Moscow for 1833 will be found the following additional testimony, if any were required, to their value:
Le temps, qui répand de la dignité sur tout ce qui échappe à son pouvoir destructeur, fait voir ici un exemple singulier de son influence ; ces substances si viles dans leur origine, étant rendues à la lumière après tant de siècles, deviennent d'une grande importance, puis qu'elles servent à remplir un nouveau chapitre dans l'histoire naturelle du globe.
Professor Agassiz had come to the conclusions which we have above endeavoured shortly to explain, from the study of more than six hundred fossils on the Continent. His visit to England has already furnished him with two hundred and fifty new species, which entirely corroborate his views. In his 'Rapport sur les Poissons Fossiles découverts en Angleterre,' our ichthyologist speaks with the most grateful acknowledgment of the facilities every where afforded him. The doors of all the museuins, both public and private, flew open at his approach. Edinburgh, with its new and interesting fossils from the limestone of Burdie-house, discovered by the zealous and discerning Dr. Hibbert-our own British Museum, whose stores were laid open to him in the most unreserved manner*--the Geological Museum at Oxford, which, under the fostering care of Dr. Buckland, has become so richBristol — York,-in short every public collection placed its specimens at his disposal. Of the private museums, those of the fossil zootomist Mantell, and of Lord Cole and Sir Philip Egerton, the one at Florence Court, the other at Oulton Park, appear to have
* We take this opportunity of observing that our own pursuits often lead us to our great national collection, and that we have always been inet by the most willing endeavours to assist in our inquiries, and by the inost obliging atteutions on the part of its officers.