138 EIGHTH REPORT— 1838. 



and are the centres from which the true calcigerous tubes radiate, and 

 they are, therefore, analogous to the simple pulp-canal of the human 

 incisor, which, with its radiating microscopic calcigerous tubes, may be 

 compared to a single medullary canal with its corresponding microsco- 

 pic radiating tubes in the Rays, Orycterojyus, &c. 



3Iyliobatis. — A longitudinal section of a single'dental plate, viewed 

 by a low power of an inch focus, exhibits at its base a coarse network. 

 of large irregular canals, filled with a vascular medullary pulp. From 

 this network smaller medullary canals proceed in a slightly-diverging 

 course, subdividing dichotomously with interspaces equal to six or eight 

 of their own diameters. In a transverse section of the tooth, seen under 

 the same power, the area of the medullary canals is seen to present ge- 

 nerally an elliptical form, from which radiating calcigerous tubes are 

 /aintly perceptible. Each canal and its series of tubes is surrounded 

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

 boundary between contiguous canals and tubes, the whole tooth being 

 thus composed of an aggregate of simple elongated, commonly six-sided 

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

 through the roots of the tooth shows that these parts are occupied by a 

 network of irregular canals, which anastomose by arched branches 

 with the network of the contiguous root, and these with the network 

 of coai-ser tubes which occupy the basis of the tooth for an extent ex- 

 ceeding the length of the root itself. 



With a higher power, ^^^th inch focus, the calcigerous tubes are seen 

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

 throughout the whole course of the canal. The 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 medul- 

 lary canal has been filled up or diminished by the deposition of a 

 series of concentric lamellae. 



The ramification of the tubes in this tooth jDresents the same general 

 character as those of Acroclus, but they are shorter, and each group in 

 the transverse section is separated from the contiguous one by the re- 

 gular boundary lines above-mentioned, which distinguish the teeth of 

 the Myliohatis from those of the Acrodus, Psammodiis, Cestracion, or 

 any of the shark tribe. The tooth of the Orycteropus is that which 

 has the nearest resemblance to the tooth of the Myliohatis. 



Acrodus nohilis. — The crushing teeth of this extinct genus are com- 

 posed of two substances, viz. a thin external almost colourless layer, 

 which represents the enamel, and an amber-coloured coarser ivory 

 composing the body of the tooth, and continuous with and passing into 

 the coarse cellular bony basis and support of the tooth. Microscopic 

 sections of tliis tooth aflx)rd the most beautiful appearances, and, per- 

 haps, the most instructive illustration of the relation of ivory to bone. 

 The body of the tooth consists of groups of beautifully branched and 

 irregularlj'^ wavy medullary canals imbedded in a clear matrix. These 

 canals are surrounded by concentric strata, and closely resemble the 

 canals of Havers in true bone. The calcigerous tubes, which radiate 

 from the medullary canals, have a graceful undulatory course and are 



