The dental plate consists, as in the Cod and Sphyreena, of a central mass of coarse osseous substance, traversed by large and nearly parallel medullary canals, and an ex ternal sheath of very hard "vitro-dentine." The medullary canals are continued from a coarse reticulation of similar but wider canals in the substance of the supporting bone, and advance forwards, nearly parallel with each other and with the plane of the upper surface of the tooth ; they anastomose together by short, curved, transverse canals, which inter cept spaces increasing in length as the canals recede from the osseous basis. The canals themselves diminish in size in the same ratio ; and when they have arrived near the dense outer layer, their divisions and inosculations become again more frequent, the peripheral loops forming a well-marked line of demarca tion between the coarse-tubed and the fine tubed dentine. The interspaces of the medullary canals are occupied by a clear sub stance, and by moss-like reticulations of fine dentinal tubes, which appear to be more sparing in number than in the teeth of the Sphyreena or Shark. The dentinal tubes of the vitro-dentine run nearly parallel to each other, and vertically to the external surface of the dental plate through about two-thirds of the thickness of that tissue ; they then bend and cross each other in a manner very similar to those of the vitro-dentine in the teeth of the Lepidotus, Phyllodus, Sze.* In the process of attrition this external dense substance is worn away from the upper surface of the dental processes in the lower jaw, exposing the softer vaso-dentinal sub stance of the tooth ; in this state the den tal plate offers an analogy to the incisors of the Rodents, a posterior softer substance being sheathed by an anterior denser layer; and an external sharp edge is similarly kept up by the unequal wearing away of the two substances. The progressive waste at the upper surface of the dental plate would appear to be met by a corresponding additon of new material to its lower part.
In the structure here presented we have a condition of the dentine which has hitherto been met with only in the class of fishes.
The test of the affinities of the present paradoxical afforded by the micro scopic examination of the teeth, gives addi tional confirmation to the view to which I have been led, from arguments drawn from the rest of its organisation, that the Lepido siren is in every essential point a member of the class of fishes.-1 If we compare the dental system of the foregoing Batrachoid fish with that in the true Batrachia, it is only to the larval state of the Anourans that an analogy can be found ; the tadpole of the frog having its maxilla and mandibula each sheathed with a single and continuous horny dental trenchant covering. Were this sheath actually dentinal in tissue and united to the jaw-bone, the re semblance to the Lepidosiren would be closer; but in point of fact the analogy is very remote ; the horny beak of the tadpole is never calcified or anchylosed, but is shed during the progress of the metamorphosis.*
The Siren alone, among the larval-like pe rennibranchiate reptiles, retains the sheath upon the extremity of the upper and lower jaws ; it consists of a firm albuminous tissue, and becomes harder than horn. But these trenchant mandibles, which play upon one another like the blades of a pair of curved scissors, are associated with numerous small but distinct true teeth, which are grouped together to form a rasp-like surface on each half of the divided vomer, and which beset the alveolar border of the splenial element of the mandible below.
In the class Reptilia, the whole order of Chelonia is edentulous, as well as the whole family of Toads (Bufmidce) in the order Batrachia; certain extinct genera of Sau rians were likewise edentulous, e. g. the re markable " Rhynchosaurus" of the new red sandstone of Shropshire, and some of the extinct Saurian of South Africa.
In the tortoises and turtles the jaws are covered by a sheath of horn, which in some species is of considerable thickness and very dense ; its working surface is trenchant in the carnivorous species, but variously sculptured, and adapted for both cutting and bruising in the vegetable feeders ; it may be said that the transitory condition of the mandibles of the Batrachian larva is here persistent.
The development of the continuous horny maxillary sheath commences, as in the parrot tribe, from a series of distinct papillae, which sink into alveolar cavities, regularly arranged (in Trionyr) along the margins of the upper and lower jaw-bones : these alveoli are in dicated by the persistence of vascular canals long after the originally separate tooth-like cones have become confluent, and the horny sheath completed.
The teeth of the dentigerous Saurian, Ophidian, and Batrachian reptiles, are, for the most part, simple and adapted for seizing and holding, but not for dividing or masticating their food. The Siren alone combines true teeth with a horny maxillary trenchant sheath, like that of the Chelonian reptiles.
With respect to number, in no existing rep tile are the teeth reduced so low as in certain mammals and fishes ; nor, on the other hand, are they ever so multiplied as in many of the latter class. The extinct Dicynodont reptiles of South Africa had hut two teeth, which were long tusks implanted in the upper jaw.* Some species of Amphisbcena (A. alba), with fifteen teeth in the upper jaw and four teen in the lower jaw, and certain Monitors (Varanus), with sixteen teeth in the upper and fourteen in the lower jaw, afford examples of the smallest number of teeth amongst existing reptiles ; and certain Batrachians, with teeth "en carries" at the roof of the mouth, or which have upwards of eighty teeth in each lateral maxillary series, present the largest number. It is rarely that the number of the teeth is fixed and determinate in any reptile so as to be characteristic of the species, and still more rarely have the individual teeth such characters as to be determined homo logically from one species to another.