STRUCTURE. 11 



the saw-fish, fPristisJ, the tesselated teeth of the eagle-rays, {Mylio- 

 bates, Zygohates, &,'c.), and the maxillary plates of the chimse- 

 roids. The dense dental case of the jaws of the parrot-fishes, 

 {Scarus), may likewise be regarded as an extreme instance of 

 this modification, and we shall find the same structure re-ap- 

 pearing in some of the inferior genera of the mammiferous class. 

 In the parrot-fishes, the denticles are quite distinct from one 

 another, but in the saw -fish, chimsera, and eagle-rays, the contiguous 

 medullary canals occasionally anastomose together. In the chimee- 

 roid fishes these anastomoses are more numerous, and the boundaries 

 of the component denticles less distinct, so that they form a 

 transition between the preceding, and what may be regarded as the 

 second variety of the tubular structure. 



In this modification, the substance of the tooth is traversed by 

 medullary canals, somewhat less regularly equidistant and less 

 parallel than in the first ; having the boundaries of their respective 

 systems of radiated calcigerous tubes indicated by the minute 

 calcigerous cells, with which the terminal branches of those tubes 

 communicate ; these boundaries being more or less obscured by 

 the terminal branches of the calcigerous tubes extending across 

 into the interspaces of the corresponding branches of an adjoining 

 system of tubes, and anastomosing with them immediately, or 

 through intervening dilatations or cells. The medullary canals 

 here dichotomize more frequently than in the first modification ; 

 their anastomoses are more numerous, and the whole tooth, which 

 is generally of large size, is consequently more individualized and 

 compacted. The teeth of the Port-Jackson shark {Cestracion Phillippi,) 

 afford a good example of this modification, which also prevails in 

 those of the extinct genera Ptychodus, Psammodus, Helodus, Ctenop- 

 tychius, Sfc. In the teeth of the extinct Acrodus, the medullary 

 canals, which likewise traverse in great numbers the body of the 

 tooth, assume a more or less wavy course ; and this disposition, 

 combined with their numerous anastomoses, leads to the third modifi- 

 cation, which at the same time is the most common and characteristic 

 of the dental structure, in the class of fishes. 



In teeth manifesting this variety of the tubular structure, the 

 dentine is permeated by a network of medullary canals, of which the 



