A still later form, coming from the Lower and Middle Eocene strata, is the genus Stylinodon. S. cylindrifer, which is the more archaic of the two described species, is only known from a single molar, fragments of a canine, and "some inconsiderable pieces of the skull." The molar is interesting on account of the fact that the enamel is still further reduced; it is represented only by narrow vertical strips, which are much narrower than those of older forms of Ganodonts. It is also hypselodont, and has a persistent pulp. So, too, the canine which had a thick anterior facing of enamel. The later species, S. mirus, is more fully known. The teeth seem to have been much the same as in the last-described species; the premolars and molars were seven in all in the lower jaw, and the canine was imbedded in the bone for a long distance, as in Calamodon. The cervical vertebrae have short centra as in Hemiganus. The clavicles were well developed. The humerus possessed an entepicondylar foramen, and its head displays the pyriform pattern so characteristic of later Edentates. The foot is clearly like that of Psittacotherium.
In reviewing the series, therefore, we see a gradual diminution of the incisors, a gradual loss of enamel on the teeth generally, and the production of hypselodont teeth growing from persistent pulps; all of which are features of the later Edentates. The progression is so gradual that the forms enumerated and described seem to have been part of a continuous series culminating in the Ground Sloths of later times. The other points of similarity will be gathered from the facts given in the foregoing pages.
There is another family belonging to the Ganodonta whose position with regard to the Edentata is not so clear. This is the family Conoryctidae, of which two genera are known. The earliest of these, from the Lower Puerco, is Onychodectes. In O. tissonensis the skull is long and narrow, thus contrasting with that of the last family. The facial part is also long. The lower jaw is much more slender. The molar formula was complete, but there is some doubt as to the incisors. The molars are tritubercular.
The other known genus is Conoryctes. Its skull has a shorter
facial portion, and is thus more like that of Stylinodontidae than that of Onychodectes. The dental formula is known, and is complete save for the loss of one incisor above and below, and one premolar above. The relationship of these Ganodonts to any later forms is uncertain; but their skeletal structure is as yet by no means fully known.