TEETH OF FISHES. 381 



being the hardest element, appears in the form of elliptical trans- 

 verse ridges, inclosing the dentine and central bone : and external 

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

 denticles. 



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

 and the complicated gi'inders of the Elephant, both in form, 

 structure, and in the reproduction of the component denticles in 

 horizontal succession. But in the fish the complexity of the 

 triturating surface is o;reater than in the mammal, since, from the 

 mode in which the wedge-shaped denticles of the Scarus are 

 implanted upon, and anchylosed to, the processes of the supporting- 

 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. The intestines are usually laden with a chalky 

 pulp, to which the coral dwellings have been reduced. 



Developement. — As might be supposed, by the above-defined 

 varied and predominating vascular organisation in the teeth of 

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

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

 petal metamorphosis and calcification of the pulp was determined 

 by observations made on the developement of the teeth in the 

 present class. 1 



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

 the well-marked stages through which the developement 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 papilla, 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 membrane : 

 these caps do not contract any organic connection with the papilli- 

 form matrix, but, as this is converted into dental tissue, ib. b, 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 tooth is gradually withdrawn from the extraneous protecting cap, 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. 263, a. Here, therefore, is represented the first and 

 transitory ' papillary ' stage of dental developement in Mammals : 

 and the simple crescentic cartilaginous maxillary plate, d, 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. 



In many Fishes, e. g. Lophius, Esox, the dental papilla? become 



1 i.xxxix. p. 784. 



