278 gidle;y: primitive mammalian foot 



If by the term "primitive opposability" is meant simple di- 

 vergence of the first digit, it seems to me purely assumption, 

 based on no convincing evidence, to interpret this condition as 

 denoting an arboreal habitat. This interpretation, moreover, 

 seems not warranted without the most positive proof, especially 

 when such a foot as that of Claenodon, for example, or any 

 other primitive mammalian foot of the basal Eocene types shows 

 no evidence of true opposability and, moreover, could be derived, 

 with such comparatively slight changes, directly from the primi- 

 tive reptilian type of foot. In fact it is but a short step in me- 

 chanical adaptation from the reptilian state to the type of foot 

 of which Claenodon is a somewhat advanced example. On exam- 

 ining a foot of Sphenodon or other reptile of generalized type, 

 the distinctly reptilian characteristics seem to be these: Foot 

 completely plantigrade; metapodials all more or less divergent, 

 permitting free spreading of the digits; metapodials shorter than 

 phalangeal portion of the digit; first digit shortest; fourth digit 

 longest of the series; phalangeal formula digits I to V, 2, 3, 4, 5, 

 and 3 or 4, respectively. Comparing now the type of foot under 

 discussion. The following seem to be the principal modifications 

 which mark its advance over the reptilian type: The foot has 

 remained plantigrade with the metapodials shorter than the 

 phalangeal portion of the digit and with digit IV, though rel- 

 atively shortened, still longest of the series; digits II to V are 

 drawn somewhat more closely together, but are still capable of 

 considerable spreading; the first digit, retaining more or less its 

 original reptilian position, is left more divergent than the others; 

 and the phalanges of the median two digits are reduced to the 

 mammalian number, three in each digit. 



These simple modifications seem to be purely mechanical 

 adaptations brought about in changing from the reptilian crawl- 

 ing manner of locomotion, to the mammalian walking gait, 

 and each modification may be readily explained on this hypothe- 

 sis. In the normal position a reptile or batrachian carries the 

 feet well out from the body, with the toes directed more or less 

 outward in such a manner that the short first digit is directed first 

 inward then forward toward the end of the stride, the heel re- 



