lOO PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



indicate a moderately elongate thorax and a short lumbar region. That 

 the tail was short, is made evident by the character of the sacrum. The 

 limbs are long and slender, much more so than in any other known repre- 

 sentative of the family and, in particular, the feet are much more elongate 

 than in any of the preceding genera. Limbs and feet have a strong, though 

 superficial, resemblance to the horses and, as frequently emphasized, the 

 feet are more strictly monodactyl than in any other known mammal. The 

 equine likeness is confined to the appendicular skeleton ; there is little in 

 the skull and still less in the vertebral column to suggest such likeness. 

 Species. — In strong contrast to Proterotherium, Thoathermm is not 

 characterized by great variability or by numerous species, well or ill 

 defined. Only one species is at all well known and the others which 

 have been named are founded upon very imperfect material. 



Thoatherium minusculum Ameghino. 



(Plates XI, Fig. 12 ; XII, Figs. 3-i3«; XIII, Figs, i, 2, 5, 10-18 ; XIV, XV.) 



Thoatherium minuscuhifn Amegh.; Enum. sistemat, etc., 1887, p. 19. 

 Merycodon rusticus Mercerat ; Rev. del Mus. de La Plata, T. I, 1891, p. 



467 [fide Ameghino). 

 Merycodon Damesi Mercerat ; Ibid, [fide Ameghino). 

 Rhagodon gracilis Mercerat; Ibid., p. 468 [fide Ameghino). 

 Thoatherium crepidatmn Amegh.; Rev. Argent, de Hist. Nat., T. I, 1891, 



p. 297. 

 Proterotheritmi cavum Lydekker (;?c»;^ Amegh.); Anales del Mus. de La 



Plata, T. II, 1893, p. 63. 

 Thoatherium rhabdodon Amegh.; Enum. Synopt., etc., 1894, p. 45. 



By far the most abundant and the only well known species of the genus is 

 T. minusculum. As the generic description has been taken almost entirely 

 from this species, it is unnecessary to repeat the account here. It may be 

 added, however, that unworn examples of m- show that the hypocone is 

 absent and that the very large protocone is separate from the posterior 

 transverse cingulum, though there is some variability in the pattern of 

 this tooth. T. crepidatitm and T rhabdodon differ from T minusculum 

 only in the development of the basal cingulum, a very untrustworthy 

 criterion. In the table, No. 62 is the cast of a mandible in the Ameghino 

 collection labeled " T. mimtsculum, co-type" ; No. 60, a cast of frag- 

 mentary upper and lower jaws referred to the same species ; No. 66, a cast 



