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contraction, on putrid vegetables and animal substances at the bottom; hence they have constant occasion to keep their bodies in the same inclined position as the extinct fossil Fishes, whose feeble brush-like teeth show that they also fed on soft substances in similar situations.*

The Sharks employ their tail in another peculiar manner, to turn their body in order to bring the mouth, which is placed downwards beneath the head, into contact with their prey. We find an important provision in every animal to give a position of ease and activity to the head during the operation of feeding.f

Fishes of the Magnesian Limestone, or Zechstein.

The Fishes of the Zechstein at Mansfeld and Eisleben have been long known, and are common in all collections; figures of many species are given by M. Agassiz. Examples of the Fishes of the Magnesian limestone of the north of England, are described and figured by Professor Sedgwick, in the Geol. Trans. of London, (2d Series, Vol. iii. p. 117, and PI . 8, 9, 10.) He states in this paper (p. 99,) that the occurrence of certain Corals and Encrinites, and several species of Producta, Area, Terebratula, Spirifier, &c. shows that the Magnesian limestone is more nearly allied in its zoological characters to the Carboniferous order, than to the calcareous formations which are superior to the New red sandstone. This conclusion accords with that which M. Agassiz has drawn from the character of its fossil Fishes.

* At the siege of Silistria, the Sturgeons of the Danube were observed to feed voraciously on the putrid bodies of the Turks and Russian soldiers that were cast into that river.

-j- This remarkable elongation of the superior lobe of the tail is found in every bony Fish of strata anterior to and including the Magnesian limestone ; but in strata above this limestone the tail is usually regular and symmetrical. In certain bony Fishes of the secondary period, the upper lobe of the tail is partly covered with scales, but without vertebrae. The bodies of all these Fishes also have an integument of rhomboidal body scales, covered with enamel.

No species of Fish has been found common to the Carboniferous group, and to the Zechstein or Magnesian limestone; but certain genera occur in both, e. g. the genus Palaeoniscus and Polypterus.

Fishes of the Muschelkalk, Lias, and Oolite Formations.

The Fishes of the Muschelkalk are either peculiar to it, or similar to those of the Lias and Oolite. The figure engraved at PI. 27% is selected as an example of the character of a family of Fishes most abundant in the Jurassic or Oolite formation; it represents the genus Microdon in the family of Pycnodonts, or thick-toothed Fishes, which prevailed extensively during the middle ages of Geological History. Of this extinct family there are five genera. Their leading character consists in a peculiar armature of all parts of the mouth with a pavement of thick round and flat teeth, the remains of which, under the name of Bufonites, occur most abundantly throughout the Oolite formation.* The use of this peculiar apparatus was to crush small shells, and small Crustacea, and to comminute putrescent seaweeds. The habits of the family of Pycnodonts appear to have been omnivorous, and their power of progression slow.f

Another family of these singular Fishes of the ancient world, which was exceedingly abundant in the Oolitic or Jurassic series, is that of the Lepidoids, a family still more

» PI. 27c. Fig. 3. represents a five-fold series of these teeth on the palate of Pycnodus trigonus from Stonesfield; and Fig. 2, a series of similar teeth placed on the vomer in the palate of the Gyrodus Umbilicus from the great Oolite of Durrheim, in Baden.

- j- A similar apparatus occurs in a living family of the Order Cycloids, in the case of the modern omnivorous Sea Wolf, Anarrhicas Lupus, and other recent Fishes of different families. M. Agassiz observes, that it is a common fact, in the class of Fishes, to find nearly all the modifications which the teeth of these animals present, recurring in several families, which in other respects are very different.

remarkable than the Pycnodonts for their large rhomboidal bony scales, of great thickness, and covered with beautiful enamel. The Dapedium of the lias (PL 1. Fig. 54.) affords an example of these scales, well known to geologists. They are usually furnished on their upper margin with a large process or hook, placed like the hook or peg near the upper margin of a tile; this hook fits into a depression on the lower margin of the scales placed next above it. (See PL 27, Figs. 3, 4, and PL 15, Fig. 17.) All Ganoidian Fishes, of every formation, prior to the Chalk, were enclosed in a similar cuirass, composed of bony scales, covered with enamel, and extending from the head to the rays of the tail.* One or two species only, having this peculiar armature of enamelled bony scales, have yet been discovered in the Cretaceous series; and three or four species in the Tertiary formations. Among living Fishes, scales of this kind occur only in the two genera, Lepidosteus and Polypterus. Not a single genus of all that are found in the Oolitic series exists at the present time. The most abundant Fishes of the Wealden formation belong to genera that prevailed through the Oolitic period .f

* The Pycnodonts, as well as the fossil Sauroids, have enamelled scales, but it is in the Leipidoids that scales of this kind are most highly developed. M. Agassiz'has ascertained nearly 200 fossil species that had this kind of armour. The use of such a universal covering of thick bony and enamelled scales surrounding like a cuirass the entire bodies of so many species of Fishes, in all formations anterior to the Cretaceous deposites, may have been to defend their bodies against waters that were warmer, or subject to more sudden changes of temperature than could be endured by Fishes, whose skin was protected only by such thin, and often disconnected coverings, as the membranous and horny scales of most modern Fishes.

+ The most remarkable of these are the genus Lepidotus, Pholidophorus, Pycnodus, and Hybodus.

Fishes of the Chalk Formation.

The next and most remarkable of all changes in the character of Fishes, takes place at the commencement of the Cretaceous formations. Genera of the first and second orders (Placoidean and Ganoidean,) which had prevailed exclusively in all formations till the termination of the Oolitic series, ceased suddenly, and were replaced by genera of new orders (Ctenoidean and Cycloidean,) then for the first time introduced. Nearly two-thirds of the latter also are now extinct; but these approach nearer to Fishes of the tertiary series, than to those which had preceded the formation of the Chalk.

Comparing the Fishes of the Chalk with those of the elder Tertiary formation of Monte Bolca, we find not one species, and but few genera, that are common to both.*

Fishes of the Tertiary Formations.

As soon as we enter on the Tertiary strata, another change takes place in the character of fossil Fishes, not less striking than that in fossil Shells.

* It has been already stated, that the remarkable deposite of fossil Fishes at Engi, in the Canton of Glaris, are referred by M. Agassix to the lower portion of the Cretaceous system.

Many genera of these are identical with, and others closely approximate to, the fishes of the Inferior chalk (Planer kalk) of Bohemia, and of the Chalk of Westphalia (see Leonhard and Bronn. Neues Jahrbuch, 1834.) Although the mineral character of the slate of Glaris presents, as we have before staled, an appearance of high antiquity, its age is probably the same as that of the Gault, or Speeton clay of England. This alteration of character is consistent with the changes that have given an air of higher antiquity than belongs to them, to most of the Secondary and Tertiary formations in the Alps.

The Fishes of the Upper chalk are best known by the numerous and splendid examples discovered at Lewes by Mr. Mantcll, and figured in bis works. These Fishes are in an unexampled state of perfection; in the abdominal cavities of one species (Macropoma) the stomach, and coprolites are preserved entire, in their natural place.

The fishes of Monte Bolca are of the Eocene period, and are well known by the figures engraved in the Ittiolitologia Veronese, of Volta; and in Knorr. About one-half of these fishes belong to extinct genera, and not one is identical with any existing species; they are all marine, and the greater number approach most nearly to forms now living within the tropics.*

To this first period of the Tertiary formations belong also the Fishes of the London clay; many of the species found in Sheppy, though not identical with those of Monte Bolca, are closely allied to them. The Fishes of Libanus also are of this era. The Fishes in the gypsum of Mont Martre are referred to the same period by M. Agassiz, who diners from Cuvier, in attributing them all to extinct genera.

The Fishes of Oeningen have, by all writers, been referred to a very recent local lacustrine deposite. M. Agassiz assigns them to the second period of the Tertiary formations, coeval with the Molasse of Switzerland and the sandstone of Fontainbleau. Of seventeen extinct species, one only is of an extra-European genus, and all belong to existing genera.

The gypsum of Aix contains some species referable to one of the extinct genera of Mont Martre, but the greatest part are of existing genera. M. Agassiz considers the age of this formation as nearly coinciding with that of the Oeningen deposites.

The Fishes of the Crag of Norfolk, and the superior Sub-apennine formation, as far as they are yet known,

* M. Agassiz has re-arrangcd these Fishes under 127 Species, all extinct, and 77 Genera, Of these Genera 38 are extinct, and 39 still living; the latter present 81 fossil species at Monte Bolca, and the former 46 species. These 39 living Genera appear for the first time in this formation.

VOL. I. 19

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