58 Ray Lankester, on the Tooth in Zipkius Sowerbiensis. 
calcareous material. The exceedingly small development of 
dentine is one of the most noticeable peculiarities presented 
by this tooth. The pulp-cavity is throughout occupied by a 
very dense vascular form of osteo-dentine, excepting a small 
elongated space which is left within the terminal cap of true 
dentine. This position for a residual pulp- cavity is, I be- 
lieve, quite unprecedented ; in other Cetacean teeth, in the 
tusks of Trichechus and Sirenia, which have their pulp- 
cavity to a large extent occupied by osteo-dentine, the re- 
sidual pulp-cavity is always at the base instead of at the 
crown of the tooth. To the naked eye the osteo-dentine 
filling the pulp-cavity in the Micropteron tooth presents in 
the longitudinal section a cracked irregular structure. 
Fissures run longitudinally through parts of the material, 
and ridges of a lighter colour and denser appearance than 
the surrounding parts traverse the surface in irregularly 
longitudinal and oblique directions. The fissures appear to 
be caused by the canals of the osteo-dentine ; the denser 
ridges by the lacunar but hard 66 globular ” matter into which 
the tissue surrounding the canals is in many parts converted. 
The points to be noted in the naked-eye appearances of the 
tooth and its section are — the minute size of the conical cap 
of dentine, its being imbedded in the surrounding cement, 
the thickness and irregular disposition of the cement, the 
position of the residual pulp-cavity, and the excessive de- 
velopment of dense globular matter in the dentine and also 
in the osteo-dentine filling the pulp-cavity. 
Microscopic characters of the dental tissues. — Plate V ) ? 
fig. 1, represents a portion of a transverse section of the 
tooth from near a (PL V, fig. 2), prepared by Mr. Topping, 
whilst PI. VI, fig. 2, is a smaller portion of a section taken a 
very little lower down. The most striking feature in both is 
the very large development of opaque, apparently structure- 
less material separating the cement from the dentine of the 
tooth.* It is very sharply marked off from the cement, but 
on the other side shades off into the dentine (or perhaps 
osteo-dentine in some parts of the tooth), of which it is really 
but a part. Both this globular matter and the cement pre- 
sent large circular and longitudinal fissures which were 
vascular canals. In fig. 1 the dentine is seen to merge into 
the osteo-dentine surrounding the two circular canals ; in 
fig. 2 it is cut off from the osteo-dentine by a fissure and 
deposits of globular matter, and has a very much more 
* The dentine of the narwhal’s tusk ( Monodon ) exhibits this same opacity 
— due to imperfect calcification — in a striking manner. It is also observ- 
able in many other large teeth, but less markedly. 
