Fig. 100. Upper and lower incisors, canines, and
premolars—½ natural size, after Ameghino.
Fig. 101. Milk incisors, canine, and premolars and
permanent m. 1—½ natural size, after Ameghino.
In the upper dentition, the small incisors, pitted on the crown, increase regularly in size toward the rear; and each has an external cingulum around the base. The canine is about twice the size of the adjacent incisor, and also has an external cingulum. The premolars increase regularly in size and also have at least an external cingulum. [Figure 101] shows a dentition which Ameghino described as the milk set of A. osborni. At the same time he remarks the unusual character of the deciduous canine in being two-cusped. I think this set of teeth should be interpreted as deciduous inc. 2 to deciduous pm. 4, plus the permanent molar 1. With such an interpretation, we find the incisors normal, the canine normal though not as large as in the permanent set, and the two-cusped tooth is the first milk premolar. The last tooth in the series is considerably different from the premolars and is evidently permanent molar 1, which is about the size and character of this tooth in A. scotti, much too small to belong to A. osborni. This set of milk teeth differ from the permanent teeth in that the premolars do not have the anterior, inner and posterior cingulum, characteristic of the permanent dentition.
The following measurements are taken from Ameghino:
| Upper dentition, | inc. 1 to pm. 4 | 104 mm. | |||
| Upper dentition, | premolar 2, | length | 18 mm., | width | 25 mm. |
| Upper dentition, | premolar 3, | length | 20 mm., | width | 28 mm. |
| Upper dentition, | premolar 4, | length | 23 mm., | width | 35 mm. |
| Upper dentition, | molar 1, | length | 28 mm., | width | 39 mm. |
| Upper dentition, | molar 2, | length | 37 mm., | width | 44 mm. |
| Upper dentition, | molar 3, | length | 50 mm., | width | 48 mm. |
CHAPTER XI
Astrapotheria
This group is composed of large, long limbed creatures, with a highly specialized dentition, in which the canines of the upper jaw are developed into great curved tushes, resembling those of Pyrotherium; while the canines of the lower jaw are compressed in the antero-posterior diameter and protrude laterally, like those of pigs. Upper premolars 1 and 2 are reduced or lacking, while pm. 3 and 4 are also reduced, but usually retained. The upper molars are brachydont, and have a crown very like that of the molars of homalodontotheres.
The lower incisors are small, proclivous, and set at intervals around the broad semicircle of the front of the fused lower jaws. The lower canines are permanently growing teeth, smaller than the upper canines, project laterally, and have the tips recurved. Premolars 1 and 2 are usually lacking, pm. 3 more or less reduced, and pm. 4 is a normal, short, molariform grinder. The lower molars have the same basal pattern as in Toxodonta, the crown carrying two crescents with a plump pillar in the basin of the posterior crescent, the pillar, however, being situated far forward near the anterior horn of the rear crescent.