The species is based on the two lower premolars described above. I reproduce the figure given by Ameghino. The measurements are: length of pm. 3 and 4, 10 mm.; height of mandible under pm. 4, 8 mm.
CHAPTER V
Typotheria
In the Deseado beds this group of running and hopping animals is well represented, making about 14% of the Amherst collection, and varying in size from a little larger than a rat to larger than a sheep.
The group all have the front teeth modified into cropping or gnawing types, which grow permanently from persistent pulps; and the back teeth also growing through the whole or a large part of life, and also rootless, the crowns being variously infolded to make grinding surfaces. The skull is flattened above, and abruptly truncated behind; the cranium being large and swollen, the facial portion broad above and excavated on the sides. The orbits are centrally located, of considerable size, and unbounded behind. The tympanic bulla is swollen and may be hollow or filled with cancellous tissue. This cavity of the tympanic is continued above and expands in the upper part of the squamosum, making a swollen capsule on either side of the back of the cranium. The openings of the auditory meatus are well back and in a tubular growth of the periotic which is directed back, and upward in an entirely characteristic manner. The strong paroccipital processes project far below the base of the carnium. The concave palate is wide and carried well back behind the teeth ending in two strong pterygoid processes. The mandible is deep, especially the back portion; has a slender coronoid process, and a small rounded articular condyle which would seem to indicate a forward and backward motion of the jaws. On account of the agreement with these general features, I have placed among the Typotheria the forms which Ameghino classified as Hyracoidea.
While agreeing in the above general features, there is great variation among the various forms. The first upper and lower incisor may be greatly enlarged or of normal size. There is a tendency for the third upper and lower incisor, the canines, and the first premolars to be reduced and disappear, and all intermediate grades are found. In the molars there is a regular tendency toward simplification; so that in the upper molars of the earlier forms there is a deep inner fold and a more moderate outer fold, either or both of which may disappear completely, though in one series the fold seems to have been accentuated instead of lost. The feet may be adapted to running or hopping.
In the Deseado and Santa Cruz material, four series of modifications may be distinguished which I have designated as families; (1) the Archaeohyracidae, primitive forms in which the incisors are little enlarged, with inner and outer folds on the molars, those on the inner side of the upper molars being very deep, bulla small, feet unknown; (2) Interatheriidae, first upper and lower incisors rooted and of moderate size, inflexions on both the inner and outer sides of the molars, bulla large, feet adapted to running; (3) Hegetotheriidae, incisor 1 of upper and lower dentition greatly enlarged and rootless, molars simplified, bulla large, feet adapted to running or to hopping; (4) Eutrachytheridae, large forms with the first upper and lower incisor enlarged and rootless, the upper molars with the inner fold developed and bifurcated, bulla large, feet unknown.
For comparison of the various genera, they are charted [on page 55], the dental character being used, as but few have the skeleton known, which is especially so of the earlier genera.
CHART OF TYPOTHERIA
| Age | Formula | Canines | U. Molars | Last i. molar | Toes | ||
|---|---|---|---|---|---|---|---|
| Hegetotherium | Santa | 3 1 4 3 | vestigal | no inner | 3-lobed | cleft | |
| Cruz | 3 1 4 3 | fold | |||||
| Prohegetotherium | Deseado | no inner | |||||
| fold | |||||||
| Pachyrukhos | Santa | 1 0 3 3 | lacking | no inner | 3-lobed | not | |
| Cruz | 2 0 3 3 | fold | cleft | ||||
| Propachyrucos | Deseado | slightly | 3-lobed | ||||
| 3 1 4 3 | reduced | ||||||
| Prosotherium | Deseado | 1 0 4 3 | lacking | simple | 3-lobed | slightly | pm. |
| 2 0 4 3 | inner fold | cleft | simple | ||||
| Archaeophylus | Deseado | ? 1 4 3 | rather | deep inner | |||
| large | slight fold, | ||||||
| outer one | |||||||
| Interatherium | Santa | 3 1 4 3 | vestigal | slight inner | 2-lobed | slightly | |
| Cruz | 3 1 4 3 | and outer | cleft | ||||
| folds | |||||||
| Protypotherium | Santa | 3 1 4 3 | large | deep inner | 2-lobed | slightly | closed |
| Cruz | 3 1 4 3 | and outer | cleft | series | |||
| folds | |||||||
| Argyrohyrax | Deseado | 3 1 4 3 | large | bifurcated | |||
| inner fold | |||||||
| Eutrachytherus | Deseado | 3 1 4 3 | lacking | bifurcated | 2-lobed | ||
| 2 1 4 3 | inner fold | ||||||
| Isoproedrium | Deseado | ||||||
| ? ? 4 3 | |||||||
| Archaeohyrax | Deseado | 3 1 4 3 | large | deep inner | 3-lobed | ||
| 3 1 4 3 | fold, slight | ||||||
| outer fold | |||||||
| Plagiarthrus | Deseado | 2-lobed | |||||
| ? ? 4 3 |