Isoproedrium solitarium Ameghino
loc. cit. above.
Fig. 51. Right mandible, dotted line indicated alveolus—
natural size.
I have assigned the specimen shown in [fig. 51] to this species. Alveoli of the first and second incisors show inc. 2 considerably larger than inc. 1. About the third incisor and the canine my specimen shows nothing, being broken down in that region. Pm. 1 is represented by a moderately large alveolus, 10 mm. long. Pm. 2 is 13 mm. long by 10 mm. wide, and slightly constricted medianly as far as the enamel is concerned, the furrow in either side being filled with cement. Pm. 3 is 18 mm. long by 12 wide and similar. There is only an alveolus for pm. 4, which is 23 mm. long. M. 1 is incomplete, but was about 23 mm. long by 10½ mm. wide and is distinguished by the deep outer inflexion. Each tooth present is covered with a heavy layer of enamel nearly a millimeter thick; and each of the teeth is unique in that the enamel is wanting on the anterior internal and the posterior internal corners of the teeth.
CHAPTER VI
Toxodontia
The toxodonts of the Deseado are much more varied than those of the Santa Cruz, and less so than those of the Casamayor; the teeth less hypsodont than in the Santa Cruz, and more hypsodont than in the Casamayor; are smaller than those of the Santa Cruz, and larger than those from the Casamayor. It is a group of heavy, short-limbed, nonadaptive animals, which, as time and competition progressed, gradually diminished in numbers and variety.
The ancestral type must be sought in some such a form as Henricosbornia, where the upper molars are brachydont, have the four primary cusps distinct, and the connecting crests of small size, and a cingulum moderately developed on the front and rear sides. Progress is in the line of enlarging the crests, so that, in the later forms, the two external cusps are united to make a wall; and the anterior external and the anterior internal cusps are united into the large anterior lobe; while the posterior external and the posterior internal cusps unite to make the posterior lobe. These may remain relatively simple as in Rhynchippidae[14]; or with this simple arrangement of the cusps, the cingulum may be developed into a platform around the anterior, inner, and posterior sides of the molars, as in the Isotemnidae; or, with relatively simple molars, the incisors may be specialized into caniniform-like teeth as in the Leontinidae; or secondary processes (or cristae) may develop from the wall, making the complicated teeth characteristic of Nesodontidae.
For convenience in discussing the modifications of the toxodont tooth, I have, throughout, used the nomenclature illustrated in [fig. 52], taking one of the most complicated to show the ultimate development. In the upper tooth there is, first, the external wall, from which springs the anterior lobe, always the larger lobe, and composed of the protocone and paracone of Osborn. In the rear is a smaller narrower posterior lobe, composed of the hypocone, the metacone, and the metaconule of Osborn. Between these is a large basin, which may be subdivided by two cristae into secondary bays, referred to as bays 1, 2 and 3, while the cristae are in the same way referred to as cristae 1 and 2. In some genera, the cristae are entirely wanting, in others incipient. When fully developed, they are most marked in young individuals and, as the tooth is worn, appear progressively shorter. Behind the posterior lobe, there is a variable bay, number 4 which is bounded behind by crista 3, which is apparently a development of the posterior cingulum. This last crista and bay may or may not be present.