312 Mr. 0. 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 lamiiife 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 t lengthening of tlie 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 witli what 

 is now going on in many instances, that I think the balance 

 of probability is rather in its favour as compared to the theory 

 of multiplication based on spasmodic fission \. It is, how- 

 ever, difficult to see how the relative claims of the two 

 suggestions can be adjusted, for Dr. Kiikenthal's observations 

 are equally consistent with either, and direct pala?ontological 

 evidence on the subject we can hardly hope to obtain. 



Dr. Kiikenthal's suggestion of the converse of the fission 

 process, i. e. the fusion of separate teeth, as a means wiiereby 

 the comparatively few and compound teeth of Mammals 

 might have si)rung from the many simple teeth of Reptiles, 

 strikes me, on the other hand, as being by no means so iiappy. 

 Not only is its modus opt'vandi 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. g. 0<jmorhinu><. 



t Indeed tliia process is by no means necessarily very gradual or slow, 

 for within the single genus Procavia we have both brachyodont and 

 hypsodout species, while the closely allied genera Gerbillns, Merioncf, 

 and Il//<))nbo)iii/H present us, in the order named, with a complete transi- 

 tion from brachyodont J/».!t-like teeth to perfectly hypsodont, rootless, 

 ever-growing teeth, with the lamiiuc 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. 



X 'J'he 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 S'/tia/odon, seem to be on the whole more 

 suggestive of Hssion than development by hypsodontism. 



§ Of the Dicynodontia there are either no marginal teeth at all or only 

 a single pair, while of the Theriodontia Ci/nosiic/tiis has II or 12, A-lluru- 

 snun/s 8 to 10, and Lj/comitnis 0, or 10, wiiile L'mj>edi(i.< has 14 to Ui and 

 TitfDiomchm 10 or 17 on each .«ide of each jaw. See Lvdekker, Cat. 

 F0.-.8. Kept. R. M. iv. pp. 71 101 (1S90). 



