1910.] The Titanotheres; the Stem Perissodactyls. 387 



and differ in significant details from those of other contemporary Perisso- 

 dactyls. 



(4) The upper molars, as compared with those of the contemporary 

 Perissodactyls, a})proach those of Eotitanops in respect to the strong develop- 

 ment of the mesostyle, the V-shaped paracone and metacone, and the 

 separation of the protoconule ridge from the protocone. 



(5) The lower premolars and molars are of the Pal8eosyo])id type, the 

 molar cusps being arranged in a /V\> from which they differ only in more 

 primitive characters; and they also contrast in details with those of the other 

 Perissodactyls. 



(6) The humerus, radius and ulna, so far as known, agree in essentials 

 with those of Eotitanops. 



(7) The manus is like that of Eotitanops; only still narrower. It differs 

 from the manus of Eohippns in the relative height of the carpals, prominence 

 of the hook of the scaphoid, wedge-shaped lunar, broader unciform, relatively 

 smaller digit III, etc. It differs from the manus of Heptodon, Hyrachyus, 

 etc., in other details. 



Nearly all the known characters in which Lambdotherium differs from the 

 later Titanotheres serve to ally it with the Lower Eocene forerunners of the 

 Equidre, Tapiridse and Rhinocerotidae. For example, the incisors are com- 

 pressed antero-posteriorly, the jaw is constricted back of the canines, and 

 the premolars and molars approach the Lower Eocene Perissodactyl type in 

 respect to the relatively large size and incipiently ridged condition of the 

 proto- and metaconules and in the form of all the remaining cusps. The 

 forearm and manus, which are the chief remaining parts known, were also 

 of the Lower Eocene Perissodactyl type, which is clearly distinguishable 

 from that of contemporary Condylarths and Artiodactyls. From certain 

 fragments of the skull, and from the known characters of Limnohyops pris- 

 cus Osborn (1909, p. 601), a primitive INIiddle Eocene Titanothere, it is 

 legitimate to infer that Lambdotherium shared with other early Perissodactyls 

 (e. g., Eohippus, Systemodon, Isectolophus) the following skull characters: 

 very broad forehead; large face; small cranium with postorbital con- 

 striction; high narrow occiput with low sagittal crest; nasals pointed, not 

 reaching to end of premaxillaries. 



Principal characters of the stem Perissodactyla. 



The phylogeny and natural classification of the various families of 

 Tertiary and Recent Perissodactyls is better known than is the case in many 

 other orders; first, because the principal evolution of the group took place 

 during the Tertiary period; secondly, because its center of dispersion lay 



