Dormice as well as Squirrels disported themselves in the Tertiary woods and thickets of Europe, and remains of several species of MYOXIDÆ occur in various deposits in France, Switzerland, and elsewhere, from the Upper Eocene onwards. Myoxus glis, the Garden Dormouse, has been identified with some doubt from the caves of Lunel Viel; and this is also probably the species occurring in the Belgian bone-caves, and described as Myoxus priscus by Dr. Schmerling. A species a little larger than the Dormouse occurs in Russian caves, and has received the name of Myoxus fossilis from M. Fischer; and the most striking species of all is also a Post-Pliocene form, namely, the gigantic Dormouse of Malta (M. melitensis). This animal, which seems to have been about the size of a Guinea-pig, must have been excessively abundant in Malta, for its describer, 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 LOPHIOMYIDÆ have yet been detected, but the great family MURIDÆ has left abundant evidence of its former existence. Species of the genera Mus, Arvicola, Myodes, 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 Meriones 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 (C. 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 Elomys, the latter considered by M. Aymard, its describer, to be allied to Hydromys. The American fossil Muridæ 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 representative of the SPALACIDÆ; and of the GEOMYIDÆ 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 DIPODIDÆ 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 Dipoïdes, from the “Bohnerz” of Würtemberg, is founded on a single tooth, and its position in this family is very doubtful.
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.
Of the OCTODONTIDÆ, an essentially American family at the present day, nearly all the recorded fossil forms are also American. Species of Echinomys, Loncheres, and Phyllomys were obtained by Dr. Lund from the Brazilian bone-caves, which also furnished him with the remains of a Coypu (Myopotamus antiquus), and of an allied form, Carterodon sulcidens, distinguished by its having broad incisors with longitudinal furrows and raised ridges. The latter has since been found living in South America. Another species, allied to Echinomys, is named by Lund Lonchophorus fossilis. The superficial deposits of South America have yielded the remains of two species of Ctenomys, one of which is believed to be identical with a recent species. As several species of this family now live in Africa, the occurrence in the eastern hemisphere of fossil forms belonging to it would not be surprising, but the few that have been referred to it are of very doubtful nature. M. Lartet obtained some isolated teeth from the Miocene of Sansan, which he described under the name of Myopotamus sansaniensis; and one or two other types (Aulacodon, Adelomys), from Upper Eocene and Miocene beds, are of very uncertain position.
Of the HYSTRICIDÆ, or Porcupines, remains have been obtained in both hemispheres. In the Old World traces of true Porcupines (Hystrix) are recorded from the Valley of the Arno, from the Sivaliks, the Pliocene deposits of the Auvergne, from Pikermi, and, on very doubtful evidence, from the Upper Eocene of Lamandine-basse; whilst Dr. Leidy has described two teeth from the Pliocene deposits of Dakota, as belonging to a species (Hystrix venustus) allied to the European Porcupine. This determination, if confirmed, would be of great interest, as no true Porcupine now occurs in America. Of the American type, two species of Sphingurus have been obtained from the Brazilian bone-caves; and Professor Cope records a species of the North American genus Erythizon from a similar cave in Pennsylvania.
The CHINCHILLIDÆ have left but scanty traces of their former existence. Lagostomus brasiliensis is from the Brazilian bone-caves; and Megamys patagoniensis from the Eocene sandstone of Patagonia. The latter species is founded upon a tibia and rotula, which on comparison seemed to approach most nearly to those of the Rodents of this family, and if the determination be correct it was probably one of the largest species of the order, as the tibia measures about a foot long. Amblyrhiza and Loxomylus, are two genera described by Professor Cope from bone-caves in Anguilla Island, West Indies.
The DASYPROCTIDÆ have but few fossil representatives, and the undoubted ones are all from the bone-caves of Brazil, which furnished Dr. Lund with two Agoutis and two Pacas. Of the former, one is described as Dasyprocta capreolus; the second is allied to the living D. caudata. The two species of Cœlogenys are extinct. Some teeth, found in Tertiary deposits of the Puy-de-Dôme, have been referred to Dasyprocta, but this determination is excessively doubtful. Diobroticus schmerlingi from Belgian caves has been placed with the Castoridæ.
Of the CAVIIDÆ, Dr. Lund obtained three species of the genus Cavia, and two of Hydrochœrus, from Brazilian bone-caves. Of the latter, one was allied to the existing Capybara; the other was a gigantic species, measuring about five feet in length. Dr. Leidy has described a species (Hydrochœrus æsopi) from teeth found in Post-Pliocene deposits in South Carolina; and the Pampean deposits of the same age furnished M. D’Orbigny with the remains of a Cavy (Cavia antiqua) which, however, is doubtfully distinct from the Patagonian species.