Ray Lankester, on the Tooth in Ziphius Sowerbiensis. 61 
which are of large size ( e . g. the tusks of the walrus), merely 
as a packing or strengthening for the hollow pulp-cavity. 
Osteo-dentine is therefore of low physiological importance. 
Dentine and enamel are the special tissues of the tooth, pre- 
sent the greatest amount of differentiation, and are formed 
under the most special conditions, and probably with the 
greatest expenditure of force. 
The tooth of Micropteron has only the small terminal cap 
formed of this special tooth structure, resembling in this a 
foetal human tooth, and, indeed, were such a tooth, minus its 
enamel, arrested in its development at this stage, its vascular 
pulp allowed to convert itself into osteo-dentine, and the sur- 
rounding tooth sac subsequently ossified into cement, we should 
have a miniature representative of the Micropteron tooth. 
The large development of opaque globular matter in the 
dentine and osteo-dentine of the Micropteron tooth is also 
significant of low organisation. The opacity is caused by 
the large number of interspaces left between globular and 
botryoidal masses of calcareous salts deposited in the forma- 
tion of the tooth, that is to say, it is due to imperfect calcifi- 
cation. This appearance is frequent in human embryonic 
teeth,* and is occasionally to be met with to a small extent in 
those of the adult and in very many mammalian teeth. It is, 
however, in the human subject corrected as development pro- 
ceeds, the interspaces being filled up. In Micropteron it is 
not so ; the rudimentary calcification is allowed to persist. 
For these reasons, then, I think it may be urged that we 
have in the teeth of Micropteron Sowerbiensis a rudimentary 
structure corresponding with a degraded function. 
Teeth of other recent Ziphioids ( Dolichodon ). — I had the 
good fortune to see the skull of the Ziphius ( Dolichodon ) 
Layardi described by Dr. Gray, at the British Museum, 
before it was packed to be returned to Cape Town, whence 
it came. The great curved teeth, one in each ramus of the 
lower jaw, as in Micropteron , are even more laterally com- 
pressed and flattened than in that genus; their length exceeded 
a foot, while their width was about two inches. The exterior 
surface was smoother than in the Oxford Cetacean, but of the 
same yellow colour characteristic of cement. I looked 
anxiously for a projecting tip of dentine as in Micropteron , 
and at the crown of each tooth, on the inner side, was a very 
small nob or nipple of brighter appearance than the sur- 
rounding surface, evidently corresponding with the dentinal 
cap of Micropteron . This protuberance is I find noticed by 
Dr. Gray in his catalogue of seals and whales, where a small 
figure of this very remarkable skull is given. Though, in the 
* Tomes. 
