DENDRODUS. 177 



denticle, thus seen in transverse section, is more rounded and less 

 regular than in either of the two above-cited instances. The cellular 

 cement which separates each denticle is relatively thicker. The 

 diameter of the central medullary canal is generally equal to two 

 thirds the diameter of the laminated and tubular wall of the denticle, 

 of which it forms the axis. In the peripheral structure, the radiated 

 trunks are shorter in comparison with the central cellular part than 

 in the preceding species. They are relatively wider apart from each 

 other, are less parallel in their course, their primary divisions diverge 

 at a more open angle, and their lateral branches are longer. 



The branches of the radiating canals, after dichotomising, are 

 resolved at their extremities into radiated systems of calcigerous tubes 

 terminating in a thin stratum of calcigerous cells. This stratum 

 forms the boundary of each system and thus describes an undulating- 

 line on each side of the radiated canals, which is very similar to the 

 anfractuous inflected layer of the external cement in the Labyrin- 

 thodon. In the transverse section of the Dendrodus hastatus the 

 calcigerous tubes seem to form, towards their extremities, a fine 

 net -work with open meshes, and to be fewer and at a greater relative 

 distance from each other than in the first described species. 



It is obvious that the plan of structure, as exhibited in the 

 longitudinal section of the tooth of the Dendrodus, bears considerable 

 analogy to that in the shark, and Scomberoid fishes, as the Sphy- 

 rsena, Dictyodus, &c. ; but one -cannot fail to recognize a greater 

 amount of parallelism in the medullary tubes in the Dendrodus, and 

 the systems of calcigerous tubes which diverge from the medullary 

 canals still more strikingly exhibit the diflerence which depends upon 

 their straighter and more parallel course, and which indicates the 

 higher type of structure in the teeth of the Dendrodus. These tubes, 

 in fact, instead of forming the inextricable moss-like reticulations, 

 which occupy the median interspaces of the medullary canals in the 

 Shark and Sphyraena, here run at right angles to the sides of the 

 tube from which they are continued, and parallel to each other, as 

 far as the middle of the interspace ; there they meet the extremities of 

 the opposite series of calcigerous tubes and are lost in minute cells. 

 The differences above manifested in the dental structure of the 



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