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BALASNODON 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, 0 0). 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, @), at each 
end of the fragment, would alone have prevented any one 
cognizant of the form and Fig. 227. 
structure of the Cachalot’s 
tooth, from mistaking the 
fossil for a tooth of that 
recent Cetacean. The mi- 
croscopic structure of the ¢c\ 
Felixstow fossil shows its 
near affinity to the Physeter, 
but at the same time proves 
its specific distinction. Transverse section of fossil tooth of 
The dentinal axis, (fig. Balznodon, Red-Crag. Nat size. 
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 ,}>,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- 
