78 GYMNODONTS. 



plane, the laminae are seen to be developed in two distinct lateral moieties, 

 which become anchylosed together by means of a thin median vertical 

 osseous partition at their median margins ; their lateral margins are 

 similarly anchylosed to the outer walls of the dentigerous cavity. 

 It is quite clear, as Cuvier observes, that the laminae are developed 

 successively : and that, in proportion as the anterior ones are worn 

 away, the posterior ones appear in readiness to replace them, so that 

 the due number of ridges, on the triturating surface, is always main- 

 tained. 



Nevertheless, these facts will be shown to be quite insufficient 

 to establish the theory of dental development by transudation of 

 layers. Any example of a continual reproduction of teeth in vertical 

 succession, would be as available in that point of view ; the peculiar 

 application of the tooth of the Diodon to illustrate the theory of 

 transudation, is due merely to the form of the denticles which com- 

 pose that compound tooth. 



Cuvier, in his examination of the dentition of the gymnodonts, 

 had employed the microscope so far as to detect the fine reticulate 

 impressions on one of the surfaces of the dental laminse of the 

 Diodon, which he rightly attributes to the impressions of vessels ; a 

 low power, as that of the ordinary pocket-lens, is sufficient to demon- 

 strate these markings. To examine the structure of the lamelliform 

 denticles, it is necessary to make extremely thin sections in a direction 

 vertical to their plane. A portion of such a section, seen by trans- 

 mitted light with a focus of half an inch, presents the appearance de- 

 lineated in Plate 39, fig. 2. Each plate here exhibits, instead of an 

 amorphous or sub-crystalline mass of excreted calcareous matter, a 

 series of extremely minute calcigerous tubes, occupying its whole 

 extent and having a general direction vertical to its plane. The 

 tubes are obviously wider at the lower side of the plate, and gradually 

 disappear in the clear and dense substance at the opposite surface. 

 When the thinnest and most transparent parts of the same section 

 are examined with a compound lens of J inch focus, the horizontal 

 partitions, which occupy the interspaces of the lamelliform teeth, are 

 seen to consist of a coarse cellular osseous texture, without any radiated 

 (Purkinjian) corpuscles, but similar to the texture of the rest of the 



