BAI^ENODON PHYSALOIDES. 



537 



being twice as thick in proportion to the diameter of the 

 central axis of dentine (figs. 218, 219, d). The second 

 difference was the cylindrical form of the slender axis, 

 and the mere filamentary tract of osteo-dentine in its cen- 

 tre, that substance being invariably and abundantly pre- 

 sent in the basal part of the same extent of dentine, 

 prior to its contracting to form the conical crown of 

 the Cachalot's tooth (fig. 218, o o). The length and 

 slenderness of the cylindrical axis of dentine in the Fe- 

 lixstow fossil tooth are peculiar to it ; and the section of 

 this dark-coloured dentinal pith (fig. 227, d), at each 

 end of the fragment, would alone have prevented any one 

 cognizant of the form and . 297 



structure of the Cachalot's 

 tooth, from mistaking the 

 fossil for a tooth of that 

 recent Cetacean. The mi- 

 croscopic structure of the 

 Felixstow fossil shows its 

 near affinity to the Pliyseter, 

 but at the same time proves 



its Specific distinction. Transverse section of fossil tooth of 



. . Balaenodon, Red-Crag. Nat. size. 



The dentmal axis, (fig. 



227, d,) is finely grooved longitudinally ; the margin of 

 its transverse section appearing crenate under a low mag- 

 nifying power. Viewed under a higher one by light trans- 

 mitted through a thin slice (fig. 228), its substance is seen 

 to be traversed by dentinal tubes radiating from the centre 

 to the circumference, in a plane more transverse to the axis 

 than in the Cachalot's dentine. The tubes present an 

 average diameter of 12 o 00 th of an inch, with interspaces 

 of about twice that diameter : they are thus more closely 

 packed than in the Cachalot. They are minutely un- 



