Fig. 13. Upper pm. 3-m. 3 of the left side—natural size, after Ameghino.
Upper dentition, pm. 3 to m. 3, length 50 mm.
Lower dentition, inc. 1 to m. 3, length 80 mm.
Macrauchenidae
(= Mesorhinidae Amegh.)
This family is distinguished, first, by the complete dental series in which none of the anterior teeth are developed into tushes; by the nasals being shortened, apparently in connection with the development of a proboscis; by its long neck; and by its feet being permanently tridactyl, all the three toes being equally developed. In the Deseado it is infrequent, but to it Ameghino has assigned two genera; Protheosodon, which he describes as similar to Theosodon, but which I find much nearer to the Casamayor representatives of this family, such as Lambdaconus, though it doubtless belongs to the series which is represented in the Santa Cruz by Theosodon. He has also made a second genus, Conioptotherium, which represents a large Macrauchenid, equal in size to Theosodon. This genus is based on the calcaleum and astragulus and seems to be rare.
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, I 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 described under the name Protheosodon. Therefore I 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 Theosodon; 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 Protheosodon 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 lumbar), the lower dentition complete, the left hind limb complete, and the right hind limb complete except for the femur. In general, the animal is about ⅗ the size of Theosodon garrettorum, but in parts varies from this as follows. The lower jaw is ⅗, the vertebrae are ⅖ in length, the hind leg is ⅗ in length but ¾ in diameter of bones, while the metatarsus is only ⅓ in length. This makes an almost plantigrade form of heavy, clumsy proportions.