48 RAYS. 



sition of a series of concentric lamellae in proportion to its term 

 of use. 



The ramification of the calcigerous tubes in this tooth, presents 

 the same general character as those of the Acrodus, but they are 

 shorter ; and each medullary canal, with the radiating series of tubes, 

 is seen in the transverse section to be separated from the contiguous 

 one, by the regular boundary lines above-mentioned, which lines distin- 

 guish the teeth of the Myliohates from those of the Acrodus, Psam- 

 modus, Cestracion, or any of the shark-tribe. The rostral teeth of 

 the saw-fish, and those of the Orycteropus, among mammalia, present 

 the nearest resemblance in their intimate structure to the teeth of the 

 Myliohates. Plate 27 gives a correct general idea of the structure of 

 the present tooth as displayed by a transverse horizontal section near 

 the root. 



The teeth of the Myliohates, like those of the rest of the plagio- 

 stomes, are successively formed at the posterior part of the tesselated 

 series in proportion as they are worn away in front. A series of minute 

 and closely aggregated papilliform matrices rise from the mucous 

 membrane behind the teeth, and are covered by a fold of the same 

 membrane which is reflected forward so as to conceal the pulps and 

 the last formed teeth. The papilliform pulps are ossified by the de- 

 position of the calcareous salts in the peripheral cells and radiating tubes, 

 but the medullary or central canal of each pulp continues to retain its 

 organizer and vascular contents till the whole of the compound tooth 

 is completed ; the calcified wall of the medullary canal is then thickened, 

 and the area diminished by the successive formation of concentric 

 laminae of osseous matter. In Zygohates, the subgenus in which I 

 have studied the development of the teeth of this family of rays, the 

 middle tooth of each transverse series is first developed ; the forma- 

 tive papillae of the two broad lateral teeth begin to rise first at the 

 mesial and anterior parts of the tooth, and from these points succeed 

 each other to the posterior and outer sides : that facet which is adapted 

 to the posterior-lateral facet of the median tooth is first completed, 

 and at this period the broad lateral tooth presents a trigonal instead 

 of a hexagonal contour, the whole length or vertical diameter of the 

 mesial side of the tooth is completed before the formation of the outer 



