CETACEANS. , 357 



tercept the light even in very thin sections, when examined in the 

 microscope by transmitted hght. 



In the Cachalot I have usually found the cement thickest 

 at the middle of the tooth, as represented in PI. 89, fig. 2, a. 

 The cement is thickest in the Dolphins at the end of the fang 

 of the old teeth, where it usually blocks up the pulp-cavity ; this 

 is similarly closed in the expanded and compressed bases of the 

 teeth of the Gangetic Platanista. 



The formation of the ordinary dentine ceases, in the teeth of 

 the Platanista, at the base of the crown, the entire expanded fang 

 being composed of tubular cement and a narrow central plate of 

 irregularly ossified pulp. A few medullary canals remain in this 

 vertical layer for the passage of the red blood into the toothy 

 whence the supply of plasma is derived for the system of minuter 

 tubes in the dentine and cement ; there are no other remains of 

 the pulp-cavity and the entire fang seems one solid mass of bone to 

 the naked eye. 



The transition from the central osseo-dentine to the cement 

 is imperceptible ; the former is distinguishable only by the grouping 

 of the concentric layers of calcigerous cells around detached vascular 

 centres and by the presence of the vascular canals. In the cement 

 these appear to be extremely few ; I saw but one in the whole extent 

 of a transverse section from the middle of the broad base of a 

 large posterior tooth of the Platanista. 



The calcigerous cells of the cement are elliptical, with the 

 extremities pointed in most, in many produced ; the long axis 

 being in the direction of the stratum ; they chiefly indicate the 

 concentric disposition of the layers of cement which follow the 

 undulations of the section or surface of the fang. The cement is 

 characterised, as in other Cetacea, by the number and parallelism 

 of the minute tubuli, which traverse it, like the tubes of the dentine, 

 in a direction vertical to its planes or layers of growth : these tubes 

 are opake and calcigerous ; are generally grouped in fasciculi with 

 clearer interspaces in which the ordinary irregular reticulating rays 

 of the elliptic cells are best seen. In each of the large or primary 

 bundles the tubuli are grouped together in smaller fasciculi \ and 



