FOSSIL ICHTHYOLOGY; COMPRISING THE SHARKS, RAYS AND OTHER PLACOID FISHES.
Lign. 184. A group of Fossil Fishes. Tertiary. Aix.
Lebias cephalotes (Agassiz).
Ascending from the two grand subdivisions of the animal kingdom, the Mollusca and the Articulata, we advance to the Vertebrata, animals distinguished from all those which have previously engaged our attention, by the possession of a bony, jointed, hollow column of support, or spine, formed of bones termed vertebræ (turn or whirl bones), and enclosing and protecting those strands or cords of the nervous system called the spinal marrow; the former classes, being destitute of such a structure, have the general name of Invertebrata.
In the beings whose mineralized remains form the subject of our present investigation, the durable parts of the frame-work, or skeleton, are, in most instances, situated internally, and their fossil relics consist principally of the bones, or solid earthy portions of their structures, either imbedded in the rocks in their natural relative position, or in a state of dismemberment and dispersion. In most cases the teeth, and in many the durable parts of their external integument, or skin, are also preserved, in a greater or less degree of integrity.
In the lowest class of vertebrata, the Fishes, the skin is covered with numerous pieces or scales, of a dense, durable substance, and strengthened, in some families, by the addition of osseous plates; thus constituting a flexible and almost impenetrable coat of armour, which affords suitable protection to beings peculiarly exposed to external injuries, from the nature of the regions they inhabit, and the state of warfare with each other in which they are constantly engaged. Confined to a fluid medium, they are provided with organs fitted for aquatic respiration, called branchiæ, or gills, and with instruments of progressive motion, termed fins, by which they are enabled to propel themselves through the water with great velocity. The apparatus for seizing, tearing, and crushing their prey presents numerous and important modifications, corresponding to the habits and economy of the different genera; their teeth offering as great variety of form and structure as those of the higher orders of animals.
The cartilaginous or the osseous nature of the skeleton, and the number and position of the fins, were the characters formerly employed in the classification of Fishes; but Prof. Agassiz, conceiving the structure of the skin to afford a natural index to the essential modifications of organization and functions, has, with great sagacity, adopted an arrangement founded upon the form and structure of the scales; and he has divided the whole class into four orders, each distinguished by essential differences in the dermal (skin) system. To the geologist this method has proved of inestimable value; for it is simple, easy of application, and, so far as our present knowledge extends, may be relied upon as affording accurate conclusions as to the nature and relations of the originals to which a few detached fossil scales may have belonged. Another important aid has been derived from the microscopical examination of the structure of the teeth; and a splendid work on this subject by Professor Owen has opened a wide field of palæontological investigation, which is yet but very partially explored.[505]
[505] Odontography; or, a Treatise on the Comparative Anatomy of the Teeth; their Physiological Relations, Mode of Development, and Microscopic Structure; illustrated by upwards of 150 plates. By Prof. R. Owen, F.R.S. &c. 4to. London. 1840-1845.
The living species of Fishes exceed eight thousand; and those found in a fossil state, and determined by M. Agassiz, already amount to upwards of one thousand five hundred; while several hundreds are still undescribed; and the rapid progress of geological research is continually adding to the number: upwards of six hundred British fossil species are enumerated. In an initiatory work like the present, it will be necessary to confine our remarks to an illustration of the mode in which the investigation of the fossil remains of the animals of this class should be conducted; and, by the elucidation of a few leading principles, prepare the student for the perusal of works expressly devoted to this branch of Palæontology.[506]