TEETH. 



879 



and teeth, that close the posterior aperture of 

 the mouth. 



The superior dentigerous pharyngeals (figs. 

 558. and 565.) present the form of an elon- 

 gated, vertical, inequilateral, triangular plate; 

 the upper and posterior margin is sharp and 

 concave ; the upper and anterior margin forms 

 a thickened articular surface, convex from side 

 to side, and playing in a corresponding groove 

 or concavity upon the base of the skull ; the 

 inferior boundary of the triangle is the longest, 

 and also the broadest; it is convex in the 

 antero-posterior direction, and flat from side 

 to side. It is on this surface that the teeth 

 are implanted, and in most species they form 

 two rows ; the outer one consisting of very 

 small, the inner one of large dental plates, 

 which are set nearly transversely across the 

 lower surface of the pharyngeal bone, and are 

 in close apposition, one behind the other : 

 their internal angles are produced beyond the 

 margin of the bone, and interlock with those 

 of the adjoining bone when the pharyngeals 

 are in their natural position ; the smaller 

 denticles of the outer row are set in the ex- 

 ternal interspaces of those of the inner row. 



The single inferior pharyngeal bone con- 

 sists principally of an oblong dentigerous 

 plate, of the form represented in Jig. 3, pi. 

 51, of my " Odontography ; " its breadth 

 somewhat' exceeds that of the conjoined 

 dentigerous surface of the pharyngeals above, 

 and it is excavated to correspond with their 

 convexity. This dentigerous plate is prin- 

 cipally supported by a strong, slightly curved, 

 transverse, osseous bar, the extremities of 

 which expand into thick obtuse processes 

 for the implantation of the triturating muscles. 

 A longitudinal crest is continued downwards 

 and forwards from the middle line of the 

 inferior pharyngeal plate, anterior to the 

 transverse bar, to which the protractor mus- 

 cles are attached. 



A longitudinal row of small oval teeth 

 alternating with the large lamelliform teeth, 

 like those of the superior pharyngt-als, bounds 

 the dentigerous plate on each side ; the inter- 

 mediate space is occupied exclusively by the 

 larger lamelliform or wedge-shaped teeth, set 

 vertically in the bone, and arranged trans- 

 versely in alternate and pretty close set series. 



The dental plates are developed in wide 

 and deep cavities in the substance of the 

 posterior part of the lower, and of the an- 

 terior part of the upper pharyngeal bones. 

 Each denticle is developed in its proper cap- 

 sule, which contains an enamel-forming pulp 

 and a dentinal pulp, in close cohesion with 

 each other and with the thin external capsule. 

 The teeth exhibit progressive stages of form- 

 ation as they approach the posterior part of 

 the upper and the anterior part of the lower 

 pharyngeal bones : as their formation advances 

 to completion they become soldered together 

 by ossification of their respective capsules 

 into one compound tooth, which soon be- 

 comes anchylosed by ossification of the den- 

 tinul pulp to the pharyngeal bone itself. 



The dentine of the pharyngeal teeth of the 



Scarus consists of calcigerous tubes and a 

 clear intermediate substance. The calcigerous 

 tubes average a diameter of ^-.th^r of an inch, 

 and are separated by interspaces equal to 

 twice their own diameter. The course of 

 these tubes is shown injfig. 558, d., in which 

 they are exposed by a vertical section through 

 the middle of two of the superior denticles. 

 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 (fg. 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, but 

 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, inclosing 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 Scarus are implanted upon, 



