TEETH OF EISIIES. 381 



being the haixlcst element, appears in tlie form of elliptical trans- 

 Tcrse ridg'cs, inclosing the dentine and central bone : and external 

 to the enamel is the cement, c, which Ijinds together the different 

 denticles. 



There is a close analogy between the dental mass of the Scarns 

 and the complicated grinders of the Elephant, both in form, 

 striieturc, and in the reproduction of the component denticles in 

 horizontal succession. But in the iish the ct.nnplexity of the 

 tiituratiiig surface is greater than in the mammal, since, from the 

 mode in which the wedge-sliapcd denticles of the Scarus arc 

 im])lanted upon, and anchylosed to, the processes of the sup)portlng 

 bone, this likewise enters into the formation of the masticatoiy 

 surface when the tooth is worn down to a certain point. 



The proof of the efficacy of the complex masticatory apparatus 

 above descriljcd is aftbrded by the contents of the alimentary 

 canal of the Scari. The intestines arc usually laden with a chalky 

 })ulp, to which the coral dwellings have been reduced. 



Dcrelopunicnt. — As might l^e supposed, by the above-defined 

 'S'aried and predominating vascular organisation in the teeth of 

 Fishes, and the jjassage from non-vascular dentine to vascular 

 dentine in the same tooth, the developement of dentine by centri- 

 petal metamorpliosis and calcification of the pulp was determined 

 by oliservations made on the developement of the teeth in the 

 present class.' 



It is interesting to observe in it the process arrested at each of 

 the well-marked stages through which the developement of a 

 ]Mamn)alian tooth passes. In all Fishes the first step is the simple 

 production of a soft ^'ascular piapilla fr(jm the free surface of the 

 buccal mendjrane : in Sharks and Rays these papiilaj, fig. 382, c, 

 do not proceed to sink into the substance of the gum, but are 

 covered by caps of an opposite free fold of the buccal memljrane : 

 these cai)s do not contract any organic connection with the papilli- 

 form matrix, but, as this is converted into dental tissue, ib. h, the 

 tooth is gradually withdrawn from the extraneous protecting cap, 

 to take its place and assume tlie erect position on the margin of 

 the jaw, fig. 2G.3, a. Here, therefore, is represented the first and 

 transitory ' papillar)' ' stage of dental developement in j\Iammals : 

 and the simple crcscentic cartilaginous maxillary plate, d, with 

 the open groove behind containing the germinal papiila3 of the 

 teeth, offers iu the Shark a magnified representation of the earliest 

 condition of the jaws and teeth in the human cinbryo. 



In manv Fishes, e. g. Loplvins, li!sox, the dental pa[)ill;e become 



' L>cxxix, p. 7S-!. 



