Page:The Geologist, volume 5.djvu/150

126 identifying the lower jaws with the forms to which they belong. No specific dissimilarity in size or position of the teeth meeting each other has been noticed by me. None of the limb-bones can be identified to belong to the same individuals as the crania or lower jaws.

I give the characters of the genus from Gervais, and the synonyms according to my own views:—

"Teeth in continuous series; certainly $7⁄7$ molars [= premolars $4—4⁄4—4$ molars $3—3⁄3—3$], four toes, of which the two median digits are the largest, and similar to each other; the two last very slender."—Gervais, ''loc. cit.''

1., Brav. memoir on Cainotherium = following species:—C. laticurvatum, Pomel, ''loc. cit. C. elegans, Pomel, l.c. C. gracile, Pomel, l.c. C. leptorhynchum'', Pomel, 'Bulletin de la Société Géologique de France,' 1846, t. iii. p. 369. C. medium, Brav., l.c. C. minimum, Brav., l.c. ? Hyægulus collotarsus, Pomel, l.c. ? Hyægulus murinus, Pomel, l.c. = C. Courtoisii, Gervais. Oplotherium laticurvatum, Laizet and De Parieu. Oplotherium leptognathum, Laizet and De Parieu. Microtherium Renggeri, H. v. Meyer. Microtherium concinnum, H. v. Meyer. Cainotherium majus, Pomel, MS. ? Cainotherium lepthelicium, Bravard, MS.?

2. Cainotherium metopium, Pomel, l.c. A very doubtful species. In British Museum?

The problem of specific creation and extinction can be best worked out upon such genera as Cainotherium. Speculation might lead a Natural-Selectionist to imagine how the four-toed Cainotheria of the Miocene, by reason of their firmer footing in a muddy soil, might have been able to go deeper in a river in quest of food, and have supplanted and caused the extinction of the Dichobunes of the Eocene, who had only one small digit at the back of their foot, making three toes in all. The differences between, e.g. Cainotherium., Dichobune, Xiphodon, and Dichodon, and Aphelotherium, are such as we might suppose might be altered through the lapse of long generations. Whether such alteration was slow and gradual by Natural Selection or any similar process, or whether it was not rather due to a more rapid and sudden method of operation, is the problem which the latter half of the nineteenth century may perhaps solve by experiment, observation, or logical demonstration. In the meanwhile, to those who bear in mind the Linnæan maxim, Omnis vera cognitio cognitione specifica innitatur, the study of the minute differences of the Cainotheria affords an instructive topic.