80 GYMNODONTS. 



The structure and formation of the compound tooth of the 

 Diodon, illustrate the nature and causes of the apparently lamellar 

 structure of the teeth of certain mammalia, especially of the conical 

 tusks, the growth of which is uninterrupted. The polished surface of 

 a vertical longitudinal section of one of these teeth exhibits con- 

 centric lines which run parallel with the outer contour of the section ; 

 these teeth, moreover, are commonly resolved by decomposition into 

 a series of superimposed laminee, or sheathed cones, and this 

 fact has been regarded as indubitable proof of their original 

 formation by successively transuded layers of dental substance. 

 Nevertheless, microscopic sections of such teeth have uniformly dis- 

 played a structure of tubes running in a direction diametrically oppo- 

 site to the plane of the lamellce. These lamellae are, in fact, due, not to 

 successive transudations from the free surface of the formative pulp, 

 but to alternations of states of comparative activity and repose in the 

 organizing processes by which the stratum of the pulp, in immediate 

 connection w^ith the last formed or basal surface of the tooth, is pre- 

 pared to receive the hardening salts, combined with alterations in the 

 nature itself of the organizing process. These alternate changes are 

 indicated by the layers of abundant calcigerous cells which are scat- 

 tered through the intertubular tissue of the tooth in planes corresponding 

 with the apparent stratification of the dental substance, and with which 

 cells angular inflections of the calcigerous tubes frequently correspond. 

 A separation of corresponding layers of parallel aggregated segments 

 of calcigerous tubes, in the tusks which manifest the preceding struc- 

 ture, is the usual result of their decomposition. 



In the compound dental plate of the Diodon, similarly parallel and 

 aggregated series of short calcigerous tubes are separated by thin layers 

 of a cellular bone ; and by decomposition, such a tooth would, in like 

 manner, exhibit the lamellated structure ; but the lamellae in this case, as 

 in the tusk of the elephant, or the conical molar of the cachalot, equally 

 present an organized structure of aggregated calcigerous tubes, di- 

 rected more or less at right angles to the plane of the lamella, and 

 indicating that higher mode of development by calcification of the 

 pulp, which it is the chief object of the present researches to ex- 

 empUfy. 



