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yet been found that is common to any two great geological formations; or living in our present seas.*
One important geological result has already attended the researches of M. Agassiz, viz. that the age and place of several formations hitherto unexplained by any other character, have been made clear by a knowledge of the fossil Fishes which they contain.f
Sauroid Fishes in the Order Ganoid.
The voracious family of Sauroid, or Lizard-like Fishes, first claims our attention, and is highly important in the physiological consideration of the history of Fishes, as it combines in the structure both of the bones, and some of the soft parts, characters which are common to the class of reptiles. M. Agassiz has already ascertained seventeen genera of Sauroid Fishes. Their only living representatives are the genus Lepidosteus,* or bony Pike (PI. 27* Fig. 1.) and the genus Polypterus (Agass. Poiss. Foss. Vol. 2. Tab. C.) the former containing five species, and the latter two. Both these genera are found only in fresh-waters, the Lepidosteus in the rivers of North America, and the Polypterus in the Nile, and the waters of Senegal.f
* The nodules of clay stone on the coast of Greenland, containing fishes of a species now living in the adjacent seas, (Mallotus Villosus) are probably modern concretions.
t Thus the slate of Engi, in the canton of Glaris, in Switzerland, has long lieen one of the most celebrated, and least understood localities of fossil Fishes in Europe, and the mineral character of this slate had till lately caused it to he referred to the early period of the Transition series. M. Agassiz has found that among its numerous fishes, there is not one belonging to a single genus, that occurs in any formation older than the Cretaceous series; but that many of them agree with fossil species found in Bohemia, in the lower Cretaceous formation, or Planer kalk; hence he infers that the Glaris slate is an altered condition of an argillaceous deposite, subordinate to the great Cretaceous formations of other parts of Europe, probably of the Gault.
Another example of the value of Ichthyology, in illustration of Geology, occurs in the fact, that as the fossil Fishes of the Wealden estuary formation are referable to genera that characterize the strata of the Oolitic series, the Wealilen deposites are hereby connected with the Oolitic period that preceded their commencement, and are separated from the Cretaceous formations that followed their termination. A change in the condition of the higher orders of the inhabitants of the waters seems to have accompanied the changes that occurred in the genera and species of inferior animals at the commencement of the Cretaceous formations.
A third example occurs, in the fact that M, Agassiz has, by resemblances in the character of their fossil Fishes, identified the hitherto unknown periods of the fresh-water deposites of Oeningen, and of Aix in Provence, with that of the Molasse of Switzerland.
The teeth of the Sauroid Fishes are striated longitudinally towards the base, and have a hollow cone within. (See PI. 27",2,3, 4; and PI. 27. 9,10,11,12,13,14.) The bones of the palate also are furnished with a large apparatus of teeth.J
PI. 27, Figs. 11, 12, 13, 14, represent teeth of the largest Sauroid Fishes yet discovered, equalling in size the teeth of the largest Crocodiles: they occur in the lower region of the coal formation near Edinburgh, and are referred by M. Agassiz to a new genus; Megalichthys. PI. 27, Fig. 9, and PI. 27*, Fig. 4, are fragments of jaws, containing many smaller teeth of the same kind. The external form of all these teeth are nearly conical, and within them is a conical cavity, like that within the teeth of many Saurians; their base is fluted, like the base of the teeth of the Ichthyosaurus. Their prodigious size shows the magnitude which Fishes of this family attained at a period so early as that of the Coal formation:* their structure coincides entirely with that of the teeth of the living Lepidosteus osseus. (PI. 27% Figs. 1, 2, 3.)
* Lepidosteus Agassiz—Lepisosteus Lacepede.
t The bones of the skull, in Sauroid Fishes, are united by closer sutures than those of common Fishes. The vertebra? articulate with the spinous processes by sutures, like the vertebra) of Saurians; the ribs also articulate with the extremities of the transverse processes. The caudal vertebra? have distinct chevron bones, and the general condition of the skeleton is stronger and more solid than in other Fishes; the air-bladder also is bifid and cellular, approaching to the character of lungs, and in the throat there is a glottis, as in Sirens and Salamanders, and many Saurians.—See Report of Proceedings of Zool. Soc. London, October, 1834.
t The object of the extensive apparatus of teeth, over the whole interior of the mouth of many of the most voracious Fishes, appears not to be for mastication, but to enable them to hold fast, and swallow the slippery bodies of other Fishes that form their prey. No one who has handled a living Trout or Eel can fail to appreciate duly the importance of the apparatus in ques. tion.
Smaller Sauroid Fishes only have been noticed in the Magnesian limestone, forming about one-fifth of the total number yet observed in this formation. Very large bones of this voracious family occur in the lias of Whitby and Lyme Regis, and its genera abound throughout the Oolite formation.* In the Cretaceous formations they become extremely rare.f They have not yet been discovered in any of the Tertiary strata; and in the waters of the present world are reduced to the two genera, Lepidosteus and Polypterus.
* We owe the discovery of these very curious teeth, and much valuable information on the Geology of the neighbourhood of Edinburgh, to the zeal and discernment of Dr. Hibbert, in the spring of 1834. The limestone in which these Fishes occur lies near the bottom of the Coal formation, and is loaded with Coprolites, derived apparently from predaceous Fishes. It is abundantly charged also with ferns, and other plants of the coal formation; and with the crustaceous remains of Cypris, a genus known only as an inhabitant of fresh-water. These circumstances, and the absence of Corals and Encrinitcs, and of all species of marine shells, render it probable that this deposite was formed in a fresh-water lake, or estuary. It has been recognised in various arid distant places, at the bottom of the carboniferous strata near Edinburgh.
In the Transactions of the Royal Society of Edinburgh, Vol. XIII. Dr. Hibbert has published a most interesting description of the recent discoveries made in the limestone of Burdie House, illustrated with engravings, from which the larger teeth in our plate are copied. (Pl. 27, Fig. 11, 12, 13,14.) The smaller figures, PI. 27, Fig. 9, and PI. 27», Fig. 4, are drawn from specimens belonging to Dr. Hibbert and the Royal Society of Edinburgh.
In this memoir. Dr. Hibbert has also published figures of some curious large scales, found at Burdie House, with the teeth of Megalichthys, and referred by M. Agassiz to that Fish. Similar scales have been noticed in various parts of the Edinburgh Coal field, and also in the Coal formation of Newcastle-on-Tyne. Unique specimens of the heads of two similar Fishes, and part of a body covered with scales, from the Coal field near Leeds, are preserved in the museum of that town.
Sir Philip Grey Egerton has recently discovered scales of the Megalichthys, with teeth and bones of some other Fishes, and also Coprolites, in the Coal formation of Silverdale, and Newcastle-under.Line. These occur in a stratum of shale, containing shells of three species of Unio, with balls of Argillaceous iron ore and plants.
Thus we see that this family of Sauroids holds a very important place in the history of fossil Fishes. In the waters of the Transition period, the Sauroids and Sharks constituted the chief voracious forms, destined to fulfil the important office of checking excessive increase of the inferior families. In the secondary strata, this office was largely shared by Ichthyosauri and other marine Saurians, until the commencement of the Chalk. The cessation of these Reptiles and of the semi-reptile Sauroid Fishes in the Tertiary formations made room for the introduction of other predaceous families, approaching more nearly to those of the present creation.J
* The Aspidorhynchus, from the Jurassic limestone of Solenhofen, (PI. 27l, Fig. 5,) represents the general character of the Sauroid Fishes.
t The Macropoma is the only genus of Sauroid Fishes yet found in the Chalk of England.
I Much light has been thrown on the history of Fishes in the Old red sandstone at the base of the Carboniferous series, by the discoveries of Professor Sedgwick and Mr. Murchison, in the bituminous schist of Caithness, (Geol. Trans. Lond. N. S. Vol. 3, part 1.;) and those of Dr. Traile, in the same schist in Orkney. Dr. Fleming also has made important observations on Fishes in the old red sandstone of Fifeshire. Farther discoveries have been made by Mr. Murchison of Fishes in the old red sandstone of Salop and Herefordshire. The general conditions of all these Fishes accord with those in the carboniferous series, but their specific details present most interesting peculiarities. Many of them will be figured by Mr. Murchison in his splendid Illustrations of the Geology of the Border Counties of England and Wales.
Fishes in Strata of the Carboniferous Order.
I select the genus Amblypterus (PL 27b.,) as an example of Fishes whose duration was limited to the early periods of geological Formations; and which are marked by characters that cease after the deposition of the Magnesian limestone.
This genus occurs only in strata of the Carboniferous order, and presents four species at Saarbriick, in Lorraine ;* it is found also in Brazil. The character of the teeth in Amblypterus, and most of the genera of this early epoch, shows the habit of these Fishes to have been to feed on decayed sea-weed, and soft animal substances at the bottom of the water: they are all small and numerous, and set close together like a brush. The form of the body, being not calculated for rapid progression, accords with this habit.
The vertebral column continues into the upper lobe of the tail, which is much longer than the lower lobe, and is thus adapted to sustain the body in an inclined position, with the head and mouth nearest to the bottom. ,
Among existing cartilaginous Fishes, the vertebral column is prolonged into the upper lobe of the tail of Sturgeons and Sharks: the former of these perform the office of scavengers. to clear the water of impurities, and have no teeth, but feed by means of a soft leather-like mouth, capable of protrusion and
* The Fishes at Saarbilck arc usually found in balls of clay ironstone, which form nodules in strata of bituminous coal shale- Lord Greenock has recently discovered many interesting examples of this, and other genera of Fishes in the coal formation at Newhaven, and Wardie, near Leith. The shore at Newhaven is strewed with nodules of ironstone, washed out by the action of the tide, from shale beds of the coal formation. Many of these ironstones have for their nucleus a fossil Amblypterus, or some other Fish; and an infinitely greater number contain Coprolites, apparently derived from a voracious species of Pygopterus, that preyed upon the smaller Fishes.