On the other hand, some fossil allies of the Dipodidæ and Geomyidæ constitute a distinct family, for which Mr. Alston proposed the name of THERIDOMYIDÆ, from that of one of its genera, _Theridomys_. In this genus, of which six species are recorded from the Eocene and Miocene deposits of France, there are four rooted molars in each series, and each of these has several enamel folds, some of which are converted into isolated loops as the crown is worn away. The best known species is _Theridomys platiceps_, from the Miocene of Caylus. In _Archæomys chinchilloides_ there are still four molars, but these present a very different structure; they are rootless, and have the enamel folds extending diagonally across the crown, so that they are composed of a series of plates, thus presenting a certain amount of resemblance to the Chinchillas, which American family _Archæomys_ was at one time supposed to represent in Europe. In fact, in the structure of their molar teeth, both the above genera approach American types; but in other characters, especially the form of the lower jaw, they appear to have been decidedly Mouse-like, and Mr. Alston regards them as most nearly related to the Dipodidæ, with which they are joined by a third form referred to the family _Issiodoromys_, a genus sometimes placed with the Jerboas. The teeth in this genus are of the same number as in the preceding, but the molars are much simpler, each of them exhibiting one large re-entering fold of enamel, which causes the surface of the tooth to present two heart-shaped lobes. This structure is not dissimilar to that prevailing in some Dipodidæ, and especially in _Pedetes_, but it was formerly thought to indicate a relationship to the Cavies, and accordingly the best-known species has received the name of _Issiodoromys pseudanæma_ (_Anæma_ being a sub-genus of Cavies). This species occurs abundantly in the Miocene lacustrine limestone near Issoire. A second species (_I. minor_) has been detected in the Upper Eocene of Lamandine-haute.