312 Mr. O. Thomas on Mammalian Dentition. 
Seals *, might easily and naturally produce a large number of 
small separate teeth, united to each other in embryonic stages 
but separate in after life. The different lamine of the 
elephant’s molars, produced, as we know, simply by hypso- 
dontism, are perfectly separate from one another until just 
before eruption, and might easily come up as separate teeth 
did the needs of the animal require it. And in the Cetacea 
the gradual + lengthening of the separate cusps, combined 
with firstly the later and later development, and finally the 
total disappearance, of the connecting ‘‘ crown,’ would be a 
modus operandi so simple and so much in accord with what 
is now going on in many instances, that I think the balance 
of probability i is rather in its favour as compared to the theory 
of multiplication based on spasmodic fissionf. It is, how- 
ever, difficult to see how the relative claims of the two 
suggestions can be adjusted, for Dr. Ktikenthal’s observations 
are equally consistent with either, and direct paleontological 
evidence on the subject we can hardly hope to obtain. 
Dr. Kiikenthal’s suggestion of the converse of the fission 
process, 7. e. the fusion of separate teeth, as a means whereby 
the comparatively few and compound teeth of Mammals 
might have sprung from the many simple teeth of Reptiles, 
Srrilees me, on the other hand, as being by no means so happy. 
Not only is its modus operandi almost inconceivable, and 
quite unlike anything that is now going on, so far as we can 
see, but it is also quite uncalled for, as the number of teeth 
in the primitive Mammalia, commonly from 14 to 16 on each 
side of each jaw, so far from being much less, is actually more 
than that found in many of the Anomodontia §, certainly the 
* E. ¢. Ogmorhinus. 
ai Indeed this process is by no means necessarily very gradual or slow, 
for within the single genus Procavia we have both brachy odont and 
hypsodont species, while the closely allied genera Gerbillus, Meriones, 
and Rhombomys present us, in the order named, with a complete transi- 
tion from brachyodont Mus-like teeth to perfectly hypsodont, rootless, 
ever-growing teeth, with the lamine entirely distinct from one another 
throughout. The close alliance of these genera in other respects shows in 
how short a period of geological time such great dental changes may take 
place. 
: { The striking fact observed by Dr. Kiikenthal of the identity in 
number of the cusps of the young compound teeth with the total number 
of the adult simple teeth is ‘decidedly in favour of the method now sug- 
gested, but, on the other hand, the appearances presented by the teeth of 
the early Cetaceans, such as Sgualodon, seem to be on the whole more 
suggestive of fission than dev elopment by hypsodontism. 
§ Of the Dicynodontia there are either no marginal teeth at all or only 
a single pair, w hile of the Theriodontia Cynosuchus has 11 or 12, A£luo- 
saurus 8 to 10, and Lycosawrus 9 or 10, while Empedias has 14 to 16 and 
Titanosuchus 16. or 17 on each side of each jaw. See Lydekker, Cat. 
Foss. Rept. B. M. iv. pp. 71-101 (1890). 
