In all fishes the teeth are shed and re newed, not once only, as in Mammals, but frequently, during the whole course of their lives. The maxillary dental plates of Lepi dosiren, the cylindrical dental masses of the Chime roid and Edaphodont fishes, and the rostra! teeth of Pristis (if these modified dermal spines may be so called) are, perhaps, the sole examples of " permanent teeth" to be met with in the whole class.
When the teeth are developed in alveolar cavities, they are usually succeeded by others in the vertical direction, as in the pharyngeal bones of the Labroids (fig. 513., art. Pisces, Vol. Ili. p.978.) ; but sometimes they follow one after the other, side by side, as in the Scaroids (PiscEs, fig. 516. c, p. 979.). The successional teeth owe the origin of their matrix to the budding out from the capsule of their predecessors of a cmcal process, in which the papillary rudiment of the dentinal pulp is developed according to the laws explained in the Introduction to my " Odontography," and the article TOOTH. But, in the great majority of fishes, the germs of the new teeth are developed, like those of the old, from the free surface of the buccal membrane through out the entire period of succession ; a cir cumstance peculiar to the present class. The Angler, the Pike, and most of our common fishes, illustrate this mode of dental repro duction ; it is very conspicuous in the carti laginous fishes, in which the whole phalanx of their numerous teeth is ever marching slowly forwards in rotatory progress over the alveolar border of the jaw, the teeth being successively cast off as they reach the outer margin, and new teeth rising from the mucous membrane behind the rear rank of the pha lanx.
This endless succession and decadence of the teeth, together with the vast numbers in which they often coexist in the same fish, illustrate the law of Vegetative or Irrelative Repetition, as it manifests itself on the first introduction of new organs in the animal kingdom, under which light we must view the above-described organised and calcified preparatory instruments of digestion in the lowest class of the vertebrate series.
At the extreme limit of the class of fishes, and connecting that class with the reptiles, stands the very remarkable genus, the dental system of which is figured in cut 560. This consists of two small, slender, conical, sharp pointed, and slightly recurved teeth, which project downwards from the nasal bone (c), and of strong trenchant dental plates anchy losed with the alveolar border of the upper (a) and lower (b) jaws, in each of which the plate is divided at the middle, or symphysial line, so as to form two distinct lateral teeth.
The office of the two laniariform teeth is to pierce and retain the nutritive substance or prey which is afterwards divided and com minuted by the strong maxillary dental plates.
The upper pair of these plates is supported by the anterior part of a strong arch of bone, which combines the characters of the su perior maxillary, palatine, and pterygoid bones ; the superior maxillary is represented by the median and anterior bar, passing in front of the dental plate of the lower jaw when the mouth is shut, terminating on each side in a process which projects outwards and back wards, as in fig. 560 , a., on each side of the an terior part of the arch ; the palatine portion constitutes the median part of the roof of the mouth behind the foregoing ; the pterygoid portion is indicated by its fulfilling the usual function of an abutment extended between the palatine portion of the upper jaw and the ar ticular pedicle of the lower jaw ; the upper dentalplates are confined to the first two parts of the arch (maxillary and palatine), and do not extend upon the pterygoid portion ; the lower dental plates (5) are anchylosed to the premandibular bone. Viewing the upper pair of plates as a single tooth, it may be described as indented at its outer surface by five vertical angular notches, penetrating in wards through half the breadth of the sup porting bone, and dividing the plate into six angular processes, which, from the direction and varying form and breadth of the entering notches, radiate from the posterior part of the median line or division of the tooth. The inferior dental plate is similarly notched on its outer side, but the proportions of the an gular indentations are such, that they receive all the processes of the upper dental plate when the mouth is shut, whilst only the four anterior processes are reciprocally received into the notches of the upper dental plate, this, with the supporting arch, being anterior to the lower plate,— a position which is deci sive in favour of its maxillary character, and against its homology with the vomer.