398 Miscellaneous. 



cMppus, A. b)'evide)is, were obtained from Pliocene deposits in 

 Oregon. Prof. Marsh thinks that Leidy's genera AncMppus and 

 Hyohippus will probably prove to be identical. 



Prof. Marsh concludes his paper with the following general 

 remarks : — 



The large number of equine mammals now known from the 

 Tertiary deposits of this country, and their regular distribution 

 through the subdivisions of this formation, afford a good oppor- 

 tunity to ascertain the probable lineal descent of the modern horse. 

 The American representative of the latter is the extinct Equus 

 fraternus, Leidy, a species almost, if not entirely, identical with 

 the Old- World Equus caballus, Linn., to which our recent horse 

 belongs, Huxley has traced successfully the later genealogy of 

 the horse through European extinct forms* ; but the line in America 

 was probably a more direct one, and the record is more complete. 

 Taking, then, as the extremes of a series, Oroliippus agilis. Marsh, 

 from the Eocene, and Equus fraternus, Leidy, from the Quaternary, 

 intermediate forms may be intercalated with considerable certainty 

 from the thirty or more well-marked species that lived in the 

 intervening periods. The natural line of descent would seem to 

 be through the following genera : — OroMppus of the Eocene ; 

 Miohippus and Ancliitherium of the Miocene ; Ancliippus, Hip- 

 parion, Protohippus, and Pliohippus of the Pliocene; and Equus, 

 Quaternary and recent. 



The most marked changes undergone by the successive equine 

 genera are as follows: — 1st, increase in size; 2nd, increase in speed, 

 through concentration of limb-bones ; 3rd, elongation of head and 

 neck, and modifications of skull. The increase in size is remarkable. 

 The Eocene Orohiqipus was about the size of a fox; Miohippwi and 

 Anchithernmi, from the Miocene, were about as large as a sheep ; 

 Hipparion and Pliohippus, of the PHocene, equalled the ass in 

 height ; while the size of the Quaternary Equus was fully up to 

 that of the modern horse. 



The increase of speed was equally marked, and was a direct 

 result of the gradual modification of the limbs. The latter were 

 slowly concentrated, by the reduction of their lateral elements and 

 enlargement of the axial one, until the force exerted by each limb 

 came to act directly through its axis, in the line of motion. This 

 concentration is well seen e. g. in the fore limb. There was, 1st, 

 a change in the scapula and humerus, especially in the latter, which 

 facilitated motion in one line only ; 2nd, an expansion of the radius 

 and reduction of the ulna, until the former alone remained entire 

 and effective ; 3rd, a shortening of all the carpal bones and en- 

 largement of the median ones, ensuring a firmer wrist ; 4th, an 

 increase in size of the third digit, at the expense of those on each 

 side, until the former alone supported the limb. The latter change 

 is clearly shown in the following diagram, which represents the 

 fore feet of four typical genera in the equine series, taken in 



* Anniversary Address, Geological Society of London, 1870. 



