FAMILY.-GADIDE.

The type-family of this order is that which includes the cod-fish, haddock, and other species of the Linnæan Gadus. The Merlinus cristatus and Rhinocephalus planiceps of the London clay, are the oldest known forms of the family; the true cod-fish (Morrhua), is not known to have existed before the present (human) period.

FAMILY.-PLEURONECTIDE.

(Flat-Fishes.)

In this family the symmetrical form is lost, and both eyes are on one side of the head. Species of still existing genera of this much-modified family have been found in tertiary deposits. The little turbot (Rhombus minimus, e. g., fig. 81)

occurs in the tertiary deposits of Monte Bolca. An equally extinct species of sole (Solea antiqua) has been found in tertiary marls near Ulm.

Char-Endo-skeleton ossified; exo-skeleton as cycloid scales; fins supported by rays, all of which, save the first in

the dorsal and pectoral of some kinds, are soft or jointed; the malacopterans are abdominal or apodal, have free operculated gills, and the swim-bladder has an air-duct.

The carp, pike, herring, salmon, eel, exemplify this order, but the species of all these genera which have left their remains in tertiary strata-and none of them are older-are distinct from the existing kinds.

The Ganoids in these formations are reduced to the genera Lepidosteus and Acipenser; but may have been represented by the palates with crushing teeth, from the Sheppey clay, to which the names Pisodus* and Phyllodus† have been given.

With respect to the fishes of the tertiary period, "they are so nearly related," says Agassiz, "to existing forms, that it is often difficult, considering the enormous number (above 8000) of living species, and the imperfect state of preservation of the fossils, to determine exactly their specific relations. In general I may say that I have not yet found a single species which was perfectly identical with any marine existing fish, except the little Capelin (Mallotus villosus), which is found in the nodules of clay of unknown geological age in Greenland." These nodules are mostly very recent, and exemplify the operation of the dissolving soft parts of the fish in consolidating the surrounding matrix.

No class of animals is more valuable in its application to the great point mooted by Uniformitarians and Progressionists than that of fishes; for their testimony is exempt from the objection on the score of the defective nature of negative evidence, to which the Progressionists' conclusions from the known genetic history of air-breathing animals may be open. It is true that many creatures living on land are never carried out to sea; but marine deposits may be expected to yield * See Owen's Odontography, p. 138, pl. 47, fig. 3. † Ibid., p. 139, pl. 47, figs. 1 and 2.

adequate grounds for determining the general character and grade of the vertebrate animals that swarmed in the seas precipitating such deposits.

We cannot, from present knowledge, assign to any past. period of the earth's history a characteristic derived from a fuller and more varied development of the entire class of fishes than has since been manifested, nor predicate of the present state of the class that it has degenerated in regard either to the number, bulk, powers, or range of modifications. of the piscine type. A retrospect of the genetic history of fishes imparts an idea rather of mutation than of progression, to which the class has been subject in the course of geological time. Certain groups, now on the wane, have formerly existed in plenary development; as, e. g., the ganoid order in the mezozoic period, and the cestraciont form of Plagiostomes in both paleozoic and mezozoic times.

As to the variety of the forms of fishes, seeing that the earth yields no indisputable evidence of Ctenoids or Cycloids anterior to the cretaceous epoch, yet still retains living representatives of both Ganoids and Placoids, the present might appear to be the culminating period in the development of fishes, in respect of the number of ordinal forms or modifications of the class. It represents, however, rather the results of mutation, depending upon the progressive assumption of a more special type, and the Scomberoids seem now to be at the head of the piscine modification of the vertebrate series. But as the retention of general vertebrate characters, in the earlier forms of fishes, implies closer affinity with the air-breathing cold-blooded class, so a higher character of organization may be predicated of the paleozoic Placoids and Ganoids than of the Ctenoids and Cycloids forming the great bulk of the class at the present day. The comparative anatomist dissecting a shark, a Polypterus, or a Lepidosteus, would point to the structures of the brain, heart, generative organs, and in the last

two genera to the air bladder and duct, as being of a more reptilian character than the corresponding parts would present in most other fishes. But the paleontologist would point to the persistent notochord, and to the heterocercal tail in palæozoic and many mezozoic fishes, as evidence of an "arrest of development," or of a retention of embryonic characters in those primæval fishes.

One other conclusion may be drawn from a retrospect of the mutations in the forms of the fishes at different epochs of the earth's history,-viz., that those species, such as the nutritious cod, the savoury herring, the rich-flavoured salmon, and the succulent turbot, have greatly predominated at the period immediately preceding and accompanying the advent of man; and that they have superseded species which, to judge by the gristly sharks and bony Garpikes (Lepidosteus), were much less fitted to afford mankind a sapid and wholesome food.

ICHNOLOGY.*

In entering upon the genetic history of the class of reptiles, we have to inquire, as in that of fishes, at what period of the earth's history the class was introduced, and under what forms; at what period it attained its plenary development, in regard to the size, grade of structure, number and diversities of its representatives; and the relations which the existing members of the class bear to its past condition. Fifteen years ago, the oldest known reptilian remains were those of the so-called "Thuringian Monitor," from the Permian copper-slates of Germany. Since that time the batrachian Apateon, or Archegosaurus has been discovered in a Bavarian coal-field; and footprints in carboniferous sandstones of North America have borne testimony to the fact, if not the * Gr., Ichnos a footstep, logos a discourse.

commencement, of reptilian existence at that period of the earth's history for, air-breathing ambulatory animals may leave other evidence of their former presence upon earth than their fossilized remains.

There are several circumstances under which impressions made on a part of the earth's surface, soft enough to admit them, may be preserved after the impressing body has perished. When a shell sinks into sand or mud, which in course of time becomes hardened into stone, and when the shell is removed by any solvent that may have filtered through the matrix, its place may become occupied by crystalline or other mineral matter and the evidence of the shell be thus preserved by a cast, for which the cavity made by the shell has served as a mould. If the shell has sunk with its animal within it, the plastic matrix may enter the dwelling-chamber as far as the retracted soft parts will permit; and as these slowly melt away, their place may become occupied by deposits of matter that had been held in solution by water percolating the matrix, and such, usually crystalline, deposit may receive and retain some colour from the soft parts of which it thus becomes the substitute.

Evidences of soft-bodied animals, such as Actinic and Medusa, and of the excremental droppings of higher animals, have been thus preserved. Fossil remains, as they are called, of soft plants, such as sea-weeds, reeds, calamites, and the like, are usually casts in matrix made naturally after the plant itself has wholly perished.

Even where the impressing force or body has been removed directly or shortly after it has made the pressure, evidence of it may be preserved. A superficial film of clay, tenacious enough to resist the escape of a bubble of gas, may retain, when petrified, the circular trace left by the collapse of the burst vesicle. The lightning flash records its course by the vitrified tube it may have constructed out of the sandy par

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