The dentine of the pharyngeal teeth of the Scams consists of calcigerous tubes and a clear intermediate substance. The calcigerous tubes average a diameter of of an inch, and are separated by interspaces equal to twice their own diameter. The course of these tubes is shown in fig. 558, d., in which they are exposed by a vertical section through the middle of two of the superior denticics. They all, on leaving the pulp-cavity, form a curve with the convexity turned towards the base of the tooth, and then bend slightly in the opposite direction ; the sigmoid curve being most marked in the calcigerous tubes at the base of the denticles, whilst those to wards the apex become longer and straighter. Besides the primary curvatures exemplified in the figure, each calcigerous tube is minutely undulated ; it dichotomises three or four times near its termination, sends off many fine lateral branches into the clear uniting substance, and finally terminates in a series of minute cells and inosculating loops at the line of junction with the enamel.
This substance (fig. 558, e.) is as thick as the dentine, and consists of a similar combination of minute tubes and a clear connecting sub stance. The tubes may be described as com mencing from the pheripheral surface of the tooth to which they stand at right angles, and, having proceeded parallel to each other halfway towards the dentine, they then begin to divide and subdivide, the branches crossing each other obliquely, and finally terminating in the cellular boundary between the enamel and dentine.
The teeth which present this complex structure are successively developed at one extremity of the bone, in proportion as they are worn away at the other ; not, however, as Cuvier describes, from behind forwards, in both upper and lower pharyngeal bones, hut in opposite directions in the opposite bones, the course of succession being from before backwards in the upper, and from behind forwards in the lower pharyngeal bones. In the progress of the attrition to which they are subjected, the thin coat of cement result ing from the ossification of the capsule is first removed from the apex of the tooth, then the enamel constituting that apex, next the den tine, and, finally, the coarse central cellular bone, supporting the hollow wedge-shaped tooth ; and thus is produced a triturating surface of four different substances of different degrees of density. The enamel, being the densest element, appears in the form of ellip tical transverse ridges, enclosing the dentine and central bone ; and external to the enamel is the cement which binds together the dif ferent denticles.
There is a close analogy between the dental mass of the Scarus and the complicated grinders of the elephant, both in form, struc ture, and in the reproduction of the compo nent denticles in horizontal succession. But
in the fish, the complexity of the triturating surface is greater than in the Mammal, since, from the mode in which the wedge-shaped denticles of the Scants are implanted upon, and anchylosed to, the processes of the sup porting bone, this likewise enters into the formation of the masticatory surface when the tooth is worn down to a certain point.
The proof of the efficacy of the complex masticatory apparatus above described is afforded by the contents of the alimentary canal of the Scari. Mr. Charles Darwin, the accomplished naturalist and geologist, who accompanied Captain Fitzroy, R. N., in the circumnavigatory voyage of the " Beagle," dissected several Parrot-fishes soon after they were caught, and found the intestines laden with nearly pure chalk, such being the nature of their excrements ; whence he ranks these fishes among the geological agents to which is assigned the office of converting the skeletons of the Lithophytes into chalk.
Development.—As might have been antici pated from the discovery of the varied and pre dominating vascular organisation in the teeth of fishes, and the passage from non-vascular dentine to vascular dentine in the same tooth, the true law of the development of dentine "by centripetal metamorphosis and calcifica tion of the cells of the pulp," was first defi nitely enunciated and illustrated from observ ations made on the development of the teeth of fishes.* It is interesting to observe in this class the process arrested at each of the well-marked stages through which the development of a mammalian tooth passes. In all fishes the first step is the simple production of a soft vascular papilla from the free surface of the buccal membrane : in Sharks and Rays these papillm do not proceed to sink into the sub stance of the gum, but are covered by caps of an opposite free fold of the buccal membrane ; these caps do not contract any organic con nection with the papilliform matrix, but, as this is converted into dental tissue, the tooth is gradually withdrawn from the extraneous protecting cap, to take its place and assume the erect position on the margin of the jaw ( fig. 510, b, a, art. PISCES, Vol. III. p. 976.). Here, therefore, is represented the first and transitory " papillary " stage of dental develop ment in Mammals ; and the simple crescentic cartilaginous maxillary plate, with the open groove behind containing the germinal papilla' of the teeth, offers in the Shark a magnified representation of the earliest condition of the jaws and teeth in the human embryo.