Teeth of Fishes.—Of all the durable parts of animals teeth occur in the mineral kingdom, the teeth of fishes present by far the most numerous, varied, and striking modifications of form, structure, composition, mode of arrangement, and attachment; and yet these dental organs, separately considered, do not in many instances, either in their structure or mode of implantation, afford characters by which the natural affinities of the original can be satisfactorily ascertained; and without the aid of other parts of the skeleton it is often impossible to determine, from external characters only, whether an unknown form of tooth belonged to an animal of the class of Fishes or of Reptiles. Although the modifications of form are almost innumerable, they are referable to four principal types; namely, the conical, the flattened, the prismatic, and the cylindrical.[508]
[508] The "Odontography" of Professor Owen should be consulted by those who would thoroughly comprehend this interesting department of science. See also the Article Teeth, by Prof. Owen in the Cyclopædia of Anatomy and Physiology.
The conical teeth are extremely variable in size and form; some are slender, almost invisible points, distributed like the pile of velvet (villous-teeth), or set like the hairs of a brush (brush-teeth); some are long and slender, or barbed at the point; others are obtuse; and many are long and striated at the base, and closely resemble the teeth of certain reptiles. The depressed teeth are equally diversified; some have the grinding surface smooth; others, deeply grooved; in some it is flat; in others convex. In form they are either lozenge-shaped, elliptical, square, oblong, semilunar, &c. The cylindrical teeth are hemispherical, or flattened; in some fishes they are short and thick; in others slender and support an obtuse, conical crown. The prismatic form is equally modified; from the compressed, sharp, lanceolate, cutting teeth, to the strong, triangular, three-pointed dentary organs.[509]
[509] For illustrations of the teeth of fishes, see [plate iv]. figs. 1, 2, 8, 10, and Ligns. [189], [191-194], [197], [198], [202], [205]; Foss. Brit. Mus. p. 449; and Ly. figs. 236, 308, 324, and 383.
The mode of arrangement and attachment of the teeth, is as diversified as their forms. In some species all the teeth are of one type, and disposed in somewhat of a serial order on both sides of the jaws; but in a large proportion of fishes there are several kinds of teeth, which are implanted not only in the jaws, properly so called, but on the bones which form the cavity of the mouth, the arches of the palate, tongue, &c.; and it is peculiar to this class of vertebrata to present examples of teeth developed in the median line (along the middle) of the mouth, as in certain species of Rays; or crossing the symphysis (the front line of union of the two sides) of the lower jaw, as in Myliobates[510] (see [Lign. 194], fig. 2). In some species the teeth are implanted in sockets, to which they are attached only by the soft parts, as in the rostral teeth of the Saw-fish; some have hollow bases, supported upon bony prominences, which rise from the base of the socket; as in several fossil teeth from the Chalk. "But by far the most common mode of attachment is by a continuous ossification between the dental pulp and the jaw,"[511] the teeth being thus anchylosed to the bone. In the Sharks the osseous bases of the teeth are attached by a ligamentous substance to the tough, dense crust, which covers the cartilaginous jaws; the teeth of these fishes are therefore generally found detached in a fossil state, in consequence of the decomposition of this substance.
[510] Odontography, p. 5.
[511] Op. cit. p. 6.
The teeth are composed of a dense, osseous material, of a finely tubular structure, termed dentine; which, in many species, forms on the external surface of the tooth a layer of firmer texture, with a glossy surface, resembling enamel. The essential character of their organization is to have a pulp or medullary cavity, or cavities, filled with a plexus of blood-vessels and nerves, from which the minute tubes composing the dentine radiate.[512] The differences observable in the size, mode of ramification, and distribution of the medullary cavities or canals, and the calcigerous tubes,[513] as revealed by microscopic exploration, constitute important distinctive characters; particularly in the examination of the fossil teeth of extinct fishes. In some teeth the dentine is traversed by equidistant, parallel, medullary canals; in others, these channels frequently subdivide, and their branches anastomose with each other. In some the medullary canals form a reticulated, or net-like structure in the dentine, the meshes of which are occupied by calcigerous tubes, and cells; often producing a dendritical appearance, as in the tooth of a fossil fish named Dendrodus. "In the highest type of structure, the dentine consists of a simple medullary cavity or canal, and a single system of calcigerous tubes, which radiate from the central or sub-central pulp-cavity, at right angles to the periphery of the tooth" (Owen), as in the teeth of the extinct Sauroid (lizard-like) fishes. A continued succession of teeth takes place during the life of the fish, and we often find in fossil specimens a series of successional teeth beneath the row in use; as in the fragment of a jaw of Lepidotus, from Tilgate Forest [Lign. 107].
[512] See Owen’s Odontography; and Tome’s Dental Physiology.
[513] Calagerous tubes; so named because they are composed of calx, or lime.