R. equinus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 463.
This species is the dominant one in the Deseado from the Chico del Chubut, west of Puerto Pisser, no less than fourteen individuals being represented in our collection, three by skulls, and one by the major part of a skeleton which was found associated with the skull of Leontinia, but was determined as belonging to this species, by the duplication of the radius with a specimen having the radius and lower jaws associated. The description of the generic characters is largely based on this skeleton. The three species are differentiated largely by their size, R. equinus being the largest, but as compared with R. pumulis it is not only larger but much heavier built.
Fig. 53. Dorsal view of skull—
½ natural size.
The skull has been described under the generic discussion. In young individuals, the furrow in the incisors and canine is marked as a groove, later as a pit, and still later is lacking altogether, as is the case in the specimen figured, for all three of my more complete specimens are old individuals, as is also Ameghino’s type. All the incisors show the cingulum near the base of the enamel. On account of the development of the cingulum on the inner anterior corner of the premolars, these teeth are broadened out and overhang the palate in a marked degree, and are also much wider than long. Molar 1 is intermediate in character between the premolars and the succeeding molars, being only slightly rhomboidal in outline, while in the last two molars the anterior external corner is markedly prolonged.
Fig. 54. Left upper
dentition,
old individual—
½ natural size.
Fig. 55. Right lower
dentition of
R. equinus;
A, of a young individual;
B, of an old individual—
½ natural size.
In different individuals, the lower jaw varies greatly in height, but this seems to me to be individual and sex variation. The three incisors and the canine are subequal in size, closely crowded, and each with a small cingulum near the base of the enamel.