GYMNODONTS. 77 



Megalichthys of minute close-set calcigerous tubes, having a diameter 

 of -rs^th of a line, with interspaces of twice that diameter. The 

 calcigerous tubes have a minutely undulated course, and pass 

 in nearly a straight line from the internal to the external surface of 

 the tooth ; the pulp-cavity extends about half-way through the body 

 of the tooth, and has a narrow elliptic transverse section ; it becomes 

 gradually smaller at the base of the tooth, and there branches out 

 into several ramifications, which are continued into the cylindrical pro- 

 cesses of the dental substance, and these are imbedded, like so many 

 piles, in the coarse osseous texture of the jaw. 



GyMNODONTS.(l) 



30. To the theory of dental development by intus-susception or the 

 deposition of calcareous tubes in the pulp's substance, as opposed 

 to that of apposition or the transudation of layers of calcareous 

 matter from the pulp's surface, may be objected the structure 

 and mode of formation of the compound teeth of the gymnodont 

 fishes, as described by Cuvier(2) and Von Born. (3) In the 

 Diodon, more especially, the lamellated structure of the tooth, and 

 its reproduction by successive transudation of layers from a per- 

 sistent pulp, were supposed to be clearly demonstrated in the 

 broad rounded triturating tubercle which is situated behind the 

 alveolar border of the jaws. The exposed surface of this tooth pre- 

 sents, in fact, a series of transverse and parallel striae (PL 38, fig. 1), 

 which, in a vertical section {ib. fig. 2) are seen to be the margins of 

 thin, superimposed, horizontal, and slightly flexuous plates, which have 

 been partially abraded by trituration in an oblique plane. The su- 

 perior layers are the most worn, and are evidently, as Cuvier 

 observed, the oldest ; in proportion as they descend, in the lower jaw, 

 they increase in breadth ; and finally, instead of being soldered 

 together, they become detached, thinner, and of a more friable tex- 

 ture, the lowest and incompletely developed plates lying loosely in the 

 cavity of the jaw beneath the superincumbent dental mass {ib. fig. 2, a). 

 If a vertical section of the tubercle be made on one side of the median 



(1) yvjjLvoc naked, odovg a tooth, fishes with teeth exposed. 



(2) Lemons d' Anatomic Compar^e, 1st and 2nd editions. 



(3) Heusinger's Zeitschrift, 1827. 



