RAYS. 45 



convexity separated by a depression on each side from a lateral and 

 less produced rising. The contour of the dentigerous surface of the 

 lower jaw presents depressions corresponding to the eminences above, 

 and vice versa. 



The structure of the tooth of the Rhina exhibits a modification 

 of the tubular structure, very different from that which characterises 

 the tooth of the extinct Ptychodus. A representation of this structure, 

 as displayed in a longitudinal vertical section of the tooth, is given in 

 PL 24, which corresponds with that of the teeth of the rays above 

 described. 



The pivot-like base of one of the component teeth of the pave- 

 ment of the jaws of the Rhina consists of a bony texture, denser than 

 that of the ordinary bone of fishes ; it is traversed by undulating, di- 

 versely directed, medullary canals, with concentrically laminated walls ; 

 numerous wavy calcigerous tubes radiate from the medullary canals, 

 subdivide in their interspaces, anastomose together, and intercom- 

 municate with numerous minute calcigerous cells. The structure 

 of this part of the tooth, we shall find to be similar to that which 

 pervades the entire tooth in another family of plagiostomous fishes. 

 The crown or body of the tooth in the Rhina exhibits, however, a 

 denser structure, and one that approaches nearer to the ordinary 

 structure of the dentine in the higher vertebrate classes. Some of 

 the medullary tubes of the base of the tooth open into the remains 

 of the cavitas pulpi, now closed up below by the ossification of the base 

 of the tooth. This cavity presents a conical form, with an oval 

 transverse section. A uniform system of calcigerous tubes radiates 

 in every direction, from the pulp-cavity towards the periphery of the 

 tooth, and vertical to that surface ; those which are continued from 

 its lower part, bend down towards the root of the tooth ; the lateral 

 ones are continued directly outwards to the sides, and the superior 

 tubes proceed vertically to the upper surface ; the intermediate 

 tubes present every gradation of oblique curvature.(l) All the tubes 

 have a minutely undulating course ; (2) they dichotomize as they 

 proceed towards the surface, and give off minute branches, generally 

 at an angle of 45° in every part of their course. These branches 

 anastomose together, and with rhomboidal calcigerous cells in the 



(1) PI. 24, fig. 1. (2)t6. fig.2. 



