LABYUINTHODONTS. 201 



receive in their interspaces corresponding vertical processes of the 

 dentine : the consequence of this disposition in maintaining a 

 grinding surface of the tooth, by the unequal attrition of the edges of 

 the interblended laminae is well known. 



The pattern, however, after which the folds of enamel and cement 

 are inflected into the substance of the tooth in these and other her- 

 bivorous species, although determinate in and characteristic of each 

 genus or species, is always more or less irregular and unsymmetrical : 

 there is no instance in the mammiferous class of these folds converg- 

 ing at regular intervals all round the circumference of the tooth towards 

 its centre. Such a disposition of the external substance of the tooth 

 may be traced at the base of the tooth in a few fishes, but is more 

 conspicuous in the fang of the tooth of the Ichthyosaur. Here, 

 the external layer of cement (for the enamel ceases at the base of 

 the crown) is inflected, at pretty regular distances round the circum- 

 ference of the tooth, towards its centre ; the vertical folds being 

 straight or plane, and extending to a distance about equal to the breadth 

 of their interspaces. These interspaces are occupied by corresponding 

 processes of the dentine, which radiate, or diverge from the central 

 mass of that substance. 



If we could suppose the tooth of the Ichthyosaur to be worn 

 down in the living animal by the masticatory uses to its complicated 

 basis, then an eighth part of the diameter of the tooth, around its 

 circumference, would present a series of ridges of the denser substance 

 converging in straight lines from that circumference. 



The plan and principle of the structure of the Labyrinthodon's 

 tooth is the same as that of the tooth of the Ichthyosaur but it is 

 carried out to the highest degree of complication. The converging 

 folds of the external cement, instead of being arrested at one fourth 

 of the distance from the circumference to the centre of the tooth, are 

 continued close to that centre ; and, instead of being simple, straight, 

 or plane lamellse, they are bent upon themselves in a series of sinuous 

 folds, resembling the anfractuosities of the brain. The ordinary 

 laws of the complication of dental structure are here, however, 

 strictly adhered to, and every space intercepted by a convolution of 

 the converging folds of the cement, is occupied by a corresponding 



