35G CETACEANS. 



the direction of the dentinal tubes at the extremity of the crown 

 of the tooth is represented in PI. 89, a, fig. 1 ; here they sooner 

 begin to diverge in graceful curves from the vertical tubes which 

 pass to the apex. The diameter of the main tubes in the Cachalot 

 is j^th of an inch, in the Platanista and Dolphin it is less. The 

 interspaces of clear substance equal from four to five of the diameters 

 of the tubes. The tubes divide dichotomously several times in their 

 course, and send off very conspicuous lateral ramuli ; they terminate 

 at the periphery of the dentine in numerous and very minute irre- 

 gularly tortuous tubes which partly anastomose together, partly 

 are lost in the contiguous cells ; the minutely undulating course, 

 the bifurcations, lateral branches, and terminal anastomoses of the 

 dentinal tubuli, and part of the peripheral layer of cells of the 

 dentine in the Cachalot's tooth is represented in PL 89 A, 

 fig. 2, a, b. As the dentine of the cetaceous tooth approximates 

 to cement by the size and number of the calcigerous cells, so the 

 cement resembles the dentine in the number and paralleUsm of 

 the fine canals, which run from its outer surface towards the dentine, 

 in the interspaces of the cells. The parallel tubes differ- from those 

 w^hich traverse the cement of the Megatherium's tooth in being too 

 minute to convey the red particles of the blood ; they scarcely 

 surpass the origins of the dentinal tubes in diameter ; the cement 

 is, however, sparingly traversed in the Cachalot by vascular or 

 medullary canals, about gi^h of an inch in diameter. In this 

 species the radiated or calcigerous cells of the cement are very 

 abundant and are arranged mostly in layers parallel with the surface, 

 having an irregularly angular outline ; a few are roundish, but 

 most are of an oval form, about aijjju th of an inch in the short 

 diameter, and ^^th of an inch in the long diameter, which is parallel 

 to the plane of the layer, (PI. 89 A, fig. 2, c.) The canals which 

 radiate from the cells, ramify and anastomose with those from 

 contiguous cells and with the branches of the parallel cemental 

 tubes. In some parts of the cement, especially near the dentine, 

 the ramifications of the tubuli are so numerous and dense, along 

 Imes parallel with the contour of the dentine, as to give the 

 appearance of white lines to the naked eye, and almost to in- 



