PLEU EtONECTIME.
(Flat-Fishea.) 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 (Rhombusminintus, e. g., fig. 63) occurs in the tertiary deposits of Monte Bolca. An equally extinct species of sole (Soles antigua) has been found in ter tiary marls near Ulm.
Fossil fishes of the cod, mullet, carp, salmon, and herring genera, are found in the tertiary formations, but are distinct from all known species. The Ganoids in these formations are reduced to the genera Lepidosteus and Aeipenser ; but may have been represented by the palates with crushing teeth, from the Sheppy clay, to which the names and Phyllodust 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 exist ing fish, except the little Capelin (Mallotus 'unions), which is found in the nodules of clay of unknown geological age in Greenland." These nodules are mostly very recent, and ex emplify the operation of the dissolving soft parts of the fish in consolidating the surrounding matrix.
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 development, to which the class has been subject in the course of geolo gical time. Certain groups, now on the wane, have existed in plenary development, as, e. g., the ganoid order in the mezo zoic period, and the cestraciont form of Plagiostomes in both palmozoic and mezozoic times.
As to the variety of the forms of fishes, seeing that the earth yields no indisputable evidence of Ctenoids or Cyeloids anterior to the cretaceous epoch, yet still retains living repre sentatives of both Ganoids and Placoids, the present would appear to be the culminating period in the development of fishes, in respect of the number of ordinal forms or modifica tions of the class. It represents, however, rather a period of
mutation of the piscine character, depending upon the pro gressive assumption of a more special piscine type, and pro gressive departure from a more general vertebrate type. The Scomberoids, as fishes, are at the head of the piscine modifica tion of the vertebrate type. But as the retention of general vertebrate characters implies closer affinity with the proximate cold-blooded class, so a higher character of organization may be predicated of the palxozoic 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, as being of a higher or a more reptilian character than the corresponding parts would present in most other fishes. But the palaeontologist would point to the persistent notochord, and to the heterocercal tail in palteo zoic and many mezozoic fishes, as evidence of an " arrest of development," or of a retention of embryonic characters in those primaeval fishes.
No class of animals is more valuable in its application to the great point now mooted by the Uniformitarian and Pro gressionists of the present day than that of fishes ; for they are exempt from the attack of the Uniformitarian on the score of the defective nature of negative evidence, to which attack conclusions from the known genetic history of air-breathing animals are open. Many creatures living on land may never be carried out to sea ; but marine deposits may be expected to yield adequate grounds for general conclusions as to the character of the vertebrate animals that swarmed in the seas precipitating such deposits.
One other conclusion may be drawn from a general retro spect 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 sal mon, 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 bony Garpikes (Lepidosteus), were much less fitted to afford mankind a sapid and wholesome food.