RAYS. 47 



which support and work these dense and heavy teeth, are pro- 

 portionally strong ; and in A^'tobatis, they nearly approach to the 

 density of true bone. In this subgenus, the upper jaw is shorter 

 and more curved than the lower one, the anterior extremity of which 

 projects beyond the upper jaw, and can be used, like a spade, in 

 digging out shell-fish, &c., from the sandy bottoms frequented by 

 these rays.(l) 



Microscopic examination of the texture of the teeth of Mylio- 

 bates has yielded the following results. A longitudinal and vertical 

 section of a single dental plate, viewed by a compound lens of an 

 inch focus (PI. 26), exhibits at its base (a) a coarse network of large 

 irregular canals, filled with a vascular medullary pulp. From 

 this network smaller medullary canals proceed, towards the flat 

 grinding surface, in a straight and slightly diverging course, 

 subdividing dichotomously with interspaces equal to their own 

 diameter at the base, but much wider at the working surface of the 

 tooth. In a transverse section of the tooth, under the same power, 

 the area of the medullary canals is seen to present generally an 

 irregular elliptical form, from which radiating calcigerous tubes are 

 faintly perceptible. Each canal and its series of tubes is surrounded 

 by a line of generally a hexagonal form, and which constitutes the 

 boundary between contiguous canals and tubes, the whole tooth being 

 thus composed of an aggregate of slender, elongated, commonly six- 

 sided prismatic teeth, placed vertically to the grinding surface. A 

 section through the roots of the tooth (b, pi. 26), shows that these 

 parts are occupied by a network of irregular canals, which anasto- 

 mose by arched branches with the network of the contiguous root, 

 and these with the network of coarser tubes which occupy the basis 

 of the tooth for an extent exceeding the length of the root itself. 



With a higher power, Jth inch focus, the calcigerous tubes are 

 seen to radiate in all directions from the medullary canals, and are 

 sent off throughout the whole course of the canal. These tubes are 

 short, wavy, richly arborescent, and form numerous anastomoses with 

 each other. The transverse sections of the tooth show that the area 

 of each medullary canal has been filled up or diminished by the depo- 



(1)P1. 16, %. 1. 



