This family name is used for the three genera Rhynchippus, Morphippus, and Eurygeniops, which made up a part of Ameghino’s family, Notohippidae. These forms I find much simpler than Coresodon, Interhippus, Stilhippus, and Nesohippus, which, by their molars, should be associated with Nesodontidae, unless it should prove that they did not have the incisors enlarged to caniniform teeth, in which case another disposition will have to be made of them. Ameghino places the Rhynchippidae among his Hippoidea, leading to horses, but we found a nearly complete skeleton of Rhynchippus equinus which in all particulars is typically toxodont. In the Deseado we found fourteen specimens belonging to this family, and strangely enough they were all Rhynchippus, and all of the species R. equinus.
This family is distinguished by the brachydont, or nearly brachydont, molar teeth, being relatively simple, and the secondary cristae not being developed. The large basin in the upper premolars and molars is, therefore, not subdivided, but is deep, and rather narrow, usually appearing as an oblique pit in the centre of the crown. There is no enlargement of the incisors to make caniniform teeth. Both the upper incisors and the canine have in the crown a longitudinal groove, which on wear becomes a pit, and being shallow may disappear entirely. The lower teeth are those typical of all toxodonts. The feet are tridactyl, and compact.
The following three genera may be distinguished:
Rhynchippus Ameghino
Rhynchippus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 462.
The teeth of both jaws are rooted, but tend to be hypsodont. The elongated incisors of the upper series are characterized by the presence of a longitudinal furrow in the top of each tooth, which, with wear, becomes a pit, and, as it is shallow, disappears in old individuals. This is the only suggestion of a horse character in the genus, but the pit in a horse’s incisor is a late development, and here it is also probably a specialization due to eating grass. Incisor 1 is the largest and they decrease in size toward either side. The canine is small, and is also marked by having a furrow in the crown, but in this case it is transverse to the long axis of the jaw.
The premolars are peculiar in having on the anterior internal corner a highly developed cingulum, which so builds out the tooth that it is usually wide and is rectangular in outline. As this cingulum rises, it incloses a bay on the ant. int. corner of the tooth, which, with wear, becomes first a bay, then a pit, and lastly may disappear entirely in old age. On each premolar the anterior and posterior lobes are developed, inclosing between them an elongated basin, which with wear becomes a long narrow pit. On the molars, the cingulum on the ant. int. corner is wanting entirely. The external anterior corner of the tooth, however, is prolonged, so that the crown has a rhomboidal outline. The crown is made up in the typical manner of the wall, the anterior, and the posterior lobes, which inclose between them an elongated basin, which, as in the premolars, becomes, on wear, a pit extending obliquely across the tooth.
The lower incisors have no furrows in the crowns, but in this genus there is a small cingulum on the inner side just above the base of the enamel. The lower canine is incisiform, and also has the basal cingulum. Each of the premolars has, on the external side, a median vertical groove, beginning at the base of the enamel, and widening toward the top. This is progressive if less marked from pm. 1 to pm. 4. The premolars and molars consist essentially of two crescents, the shorter anterior, and the posterior which is about twice as long as the anterior. The details are as described on page 96, and seen in [figure 55].
The skull is of moderate height, nearly flat on top with wide zygomatic arches. The sagittal crest is moderately high, and slightly convex in the antero-posterior direction. The occipital region is overhanging and topped by short lambdoidal crests, which, extending to either side, unite with the zygomatic arches. The nasals are large, roughly rectangular, and slightly constricted just in front of the middle. The frontals are short, and project over the orbits in strong processes. The maxilla is large, bounding the front of the orbit, and extending backward in a strong zygomatic process which makes nearly half of the arch. The jugal, while stout, is short, and reaches from the long zygomatic process of the maxilla to the short one of the squamosum. The lachrymal bone is tiny, with a low tubercle, just below which is situated the lachrymal duct, just on the margin of the orbit. Just behind the zygomatic arch, the squamosum is inflated and contains a large hollow chamber, as is typical among toxodonts. The mastoid bullae, while relatively small, are swollen into a globular form, and have a large hollow chamber. The palate extends back to just behind the last molar, a feature distinguishing this genus from Morphippus.