FOSSIL RODENTS. 153 



Guinea-pig, must have been excessively abundant in Malta, for its desciiber, Professor Leith Adams, 

 says that " its remains are met with in abundance throughout the cavern and fissure deposits, up 

 even. to the superficial alluvium now in course of formation." From older times we have evidence of 

 the existence of a Dormouse, about the size of the common species, at the time of the deposition of 

 the gypsum of Montmartre (Upper Eocene), in which a well-preserved skeleton of the animal has 

 been found. The same deposit has furnished traces of a second rather larger species. The Miocene of 

 Switzerland and of Sansan has also yielded species of Myoxus; and Professor Hermann von Meyer 

 has recorded a Dormouse from the Miocene of Weisenau, under the name of Brachymys ornatus. 



No fossil LOPHIOMYID^E have yet been detected, but the gi^eat family MURID.E has left abundant 

 evidence of its former existence. Species of the genera JJus, Arvicola, Mi/odes, and Cricetus, identical 

 in many cases with those now living, have been obtained frequently in Post-Pliocene deposits and in bone, 

 caves in Europe. Lemmings (Myodes lemmus and torquatus) are recorded from English caves. The 

 genus Mus is also represented by several species in the Miocene deposits of France, and in the Sivalik 

 beds investigated by Falconer and Cautley. The Miocene of Sansan has furnished a form which 

 has been doubtfully regarded as a Gerbille, and named M&riones Laurillardi. In the same and other 

 deposits of the same age in South-eastern France several species of an extinct genus (Cricetodon) have 

 been obtained. Their dentition resembles that of the Hamster, but the first molars in both jaws have a 

 tubercle less ; the largest species (G. sansaniensis) rather exceeded the Hamster in size, while the 

 smallest was less than a Mouse. Associated with some of these are two doubtful forms, Decticus and 

 JHomys, the latter considered by M. Aymard, its describer, to be allied to Hydromys. The American 

 fossil Muridse are for the most part either species of the genus Hesperomys, or nearly related to it. 

 Twelve species of that genus were obtained by Dr. Lund from the Brazilian, bone-caves, but of these 

 eight were identified by him with species still existing. In North America two species of a nearly- 

 allied genus (Eumys) have been obtained from Miocene deposits; and the bone-caves of Pennsylvania 

 furnish the remains of a species of Neotoma (N. magister], hardly distinguishable from the Florida Rat. 



A Rhizomys from the Sivalik deposits of North-western India is the only recorded fossil represen- 

 tative of the SPALACID^E ; and of the GEOMYID^E the only known species are a Geomys from the 

 Pliocene of Nebraska, nearly allied to, if not identical with, the living G. bursarius ; and one from the 

 " Tertiaries of the Plains," described by Professor Cope as Colotaxis cristatus, which, however, has 

 only three molars in the lower jaw. 



The DIPODID.E are still more scantily represented. A Jerboa described by M. Fischer from 

 Post-Pliocene deposits, probably of Tartary, is very nearly allied to the living Dipus platurus, but has 

 shorter toes and broader cannon-bones. The genus Dipoides, from the " Bohnerz" of Wiirtemberg, 

 is founded on a single tooth, and its position in this family is very doubtful. 



On the other hand, some fossil allies of the Dipodidae and Geomyidse constitute a distinct family, 

 for which Mr. Alston proposed the name of THERIDOMYID^E, 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 Archceomys chinchilloides there are still four molars, but 

 these present a very different structure ; they are rootless, and have the enamel folds extending dia- 

 gonally 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 Archceomys 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 Dipodidse, with which they are joined by a third form referred to the family Jssio- 

 doromys, 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 Dipodidfe, 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 pseudancema (Ancema being a sub-genus of Cavies). 



