60 Geological Society. 



conceives that the structure of the upper part of the tooth may be 

 more simple than that of the lower, but he has not yet been able 

 to extend his investigations to it. 



The dentine consists of a slender, central, conical column or 

 " modiolus," hollow^ for a certain distance from its base, and radi- 

 ating outwards from its circumference a series of vertical plates, 

 which divide into two, once or twice, before they terminate at the 

 periphery of the tooth. Eacli of these diverging and dichotomizing 

 vertical plates gives off throughout its course narrower vertical 

 plates, which stand at nearly right angles to the main plate, in rela- 

 tion to which they are generally opposite, but sometimes alternate. 

 Many of the secondary plates, which are given off near the centre 

 of the tooth, also divide into two before they terminate. They par- 

 take of all the undulations which characterize the inflected folds of 

 the cement. 



The central pulp-cavity is reduced to a line, about the upper third 

 of the tooth ; but fissures radiate from it, corresponding in number 

 with the radiating plates of the dentine. One of these fissures is 

 continued along the middle of each plate, dividing where it divides, 

 and penetrating each bifurcation and process ; the main fissures ex- 

 tend to within a line or half a line of the periphery of the tooth ; 

 the terminations of these, as well as the fissures of the lateral pro- 

 cesses, suddenly dilating into subcircular, oval, or pyriform spaces. 

 All these spaces constitute centres of radiation of the fine calcige- 

 rous tubes, which, with their uniting clear substance, constitute 

 the dentine. The number of these calcigerous tubes, which are the 

 centres of minor ramifications, defies all calculations. Their diameter 

 is the 77/uoth of a line, with interspaces equal to seven diameters of 

 their cavities. 



Mr. Owen then compares the structure of the section of a tooth 

 procured in the sandstone of Coton-End Quarry, and lent to him by 

 Dr. Lloyd of Leamington. The tooth nearly i-esembles in size and 

 form the smaller teeth of Labyrinthodon figured by Prof. Jiiger. All 

 the peculiarities of the labyrinthic structure of the Keuper tooth are 

 so clearly preserved in this specimen, that the differences are merely 

 of a specific nature. 



At the upper part of the tooth a thin layer of enamel*, besides a 

 coating of cement, is inflected at each groove towards the centre of 

 the dentine ; but about the middle of the tooth the enamel disap- 

 pears, and the convolutions consist of interblended layers of cement 

 and dentine. Thus, on the supposition that the tooth of the Laby- 

 rinthodon of the German Keuper be capped with enamel, its extent 

 must be less than in the tooth of the Warwick sandstone. 



The inflected folds are continued for a greater relative distance 

 before the lateral inflections commence than in the German species, 

 and the anfractuosities are fewer in number, and some of the folds 



* Mr. Owen has subsequently ascertained that this is not true enamel, 

 but a layer of firm dentine, separated from the rest by a thin stratum of 

 fine calcigerous cells. 



