PROF. OWEN ON HYPSIPRYMNODON. 581 



shape, and proportion best adapted to the need of this feathered denizen of the desert. 

 But the nomogenist*, in reply, asks for the significance, or an explanation, of the 

 retention of the metatarsal of the last toe (fig. 13, n), the jointless pointed extremity of 

 which projects, like the splint-bone of the same lost toe in the Horse, on the inner side 

 of the articular end of the huge metatarsal of the adjoining functional toe (in). 



In speculating on the way of operation of the secondary law in the coming to be of 

 species and genera distinguished, among other characters, by the number and pro- 

 portions of the toes, a nomogenist has hazarded the following supposition : — " as the 

 surface of the earth consolidated, the larger and more produced mid hoof of the old three- 

 toed Pachyderms \_Palceotlierium~\ took a greater share in sustaining the animal's weight ; 

 and more blood being required to meet the greater demand of the more active middle toe, 

 it grew ; whilst the side toes, losing their share of nourishment, and becoming more and 

 more withdrawn from use, shrank ; and so on, according to the hardening of the ground, 

 until only the hidden rudiments of metapodials [n and iv] remained, and one hoof became 

 maximized for all the work"t- The writer illustrates this notion by a subjoined 

 diagram of the structure of the foot in the ungulates of the Middle Eocene, the Miocene, 

 and the Pliocene periods |. 



But since the publication of the volume above cited, the structure of the foot of an 

 older ungulate mammal, the Qorypliodon of our plastic clay§, has been demonstrated by 

 the fossils of that genus discovered by Professor Marsh in the corresponding Tertiaries of 

 North America || . 



And here, as in the oldest known form of Marsupials, the primitive pentadactyle type 

 of foot is revealed, from which the successive perissodactyle hoofed forms have deviated 

 by concentration of size and force through strenuous use upon digit in, and by 

 degeneracy through opposite physiological conditions affecting size and power, first of 

 digits i and v, which are lost in JPalceotherium, next of digits n and iv, dwarfed in 

 Ancliitherium and Hipparion, until finally these also disappear in JEquus. 



An analogous modification under other functions of the hind foot is traceable in 

 certain extinct species of Bruta (Edentata, Cuv.). 



In Mylodon digit I is obsolete ; n is small, but retains the ungual phalanx ; 

 development is concentrated on in; the ungual phalanx is wanting and the others 

 stunted, in iv and v *|f . 



In Megatherium both digits i and n are gone ; iv and v are clawless, stunted, and 

 seem to have been developed in a common hoof-like callosity. The whole power of the. 

 foot as an uprooter is concentrated in digit in **. 



* ' Anatomy of Vertebrates,' vol. iii. 1868, p. 814. f Tom. cit. p. 793. + Ibid, woodcut 614. 



§ Owen, ' British Fossil Mammals,' 8vo, 1846, p. 299, cuts 103, 105. 



|| Marsh, " Principal Characters of the Goryphodontidce," American Journal of Science and Arts, vol. xiv. July 1877, 

 p. 81, pi. iv. fig. 2. A more direct and continuous ancestry of existing Equines will shortly be described and 

 illustrated, in American Fossils, by the same author. 



% Owen, ' Description of Mylodon,' 4to, 1842, p. 117, pi. xxii. 



** ' Memoir on the Megatherium,' 4to, 1860, p. 72, pis. xxv. and xxvi. The degree and course of mutilation and 

 of partial and exaggerated development of certain toes in the hind foot of other Bruta are detailed in pp. 74-77. 



SECOND SERIES. — ZOOLOGY, VOL. I. 4 H 



