PRIMITIVE MACRAUCHENID 4I 
Protheosodon Ameghino 
Protheosodon, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453. 
Protheosodon, Amegh., 1904, Anal. Mus. Nac. B.A., ser. 3, t. 3, p. 421. 
This genus was founded on an upper second molar and 
the fourth premolar. I figure m. 2, and it will be seen that 
they represent a form little specialized, resembling in the 
low crowns, plump cusps, and presence of both protoconule 
and metaconule, the Casamayor types, such as Lambdaconus 
or Didolodus, rather than the advanced type like the Santa 
Cruz genus, Theosodon. We found a specimen with the 
lower jaws complete and with the hind limb complete, 
which, | am confident, is the same form, though I can not 
duplicate any tooth, for we found no upper teeth; but in 
size they agree with Protheosodon, also in the primitive 
character; and, were one from the lower teeth to postulate 
the upper, they would be just such as Ameghino has de- 
scribed under the name Protheosodon. ‘Therefore | have 
assigned my material to this genus and species. It adds 
to the genus characters the fact that this form had a shorter 
back, relatively as well as actually, than 7Theosodon; that 
the hind limb, at least, was much heavier and also shorter 
than that of Theosodon, especially in the metatarsal region 
where relatively the elements are only about half as long. 
The pes is of the same character as in Theosodon, but again 
relatively much shorter. I believe in Prothesodon we have 
to do with a form intermediate between Lambdaconus and 
Theosodon, and nearer to the former. 
Protheosodon coniferus Ameghino. 
P. coniferus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453. 
Ameghino has described two upper teeth. Specimen 
No. 3001 of the Amherst Collection from the Chico del 
Chubut River, west of Puerto Visser, adds to this the 
knowledge of twelve vertebrae (seven dorsal and five lum- 
bar), the lower dentition complete, the left hind limb 
