FOSSIL RODENTS. 151 



CHAPTER V. 



FOSSIL RODENTIA. 



Families of Rodent, represented by Fossil Remains State of the " Record of the Rocks " THE SCIURID^E Sciurine Genera 

 now Extinct- -No Fossil ANOMALURID.E and HAPLODONTIDJE ISCHYROJIYIDJE Pseudotomas hians Gymnoptychus 

 CASTORID.E Mr. Allen's CASTOROIDID.E--THE MYOXID.E No Fossil LOPHIOMYIDJE THE MURID.E THE SPALACIDJE 

 THE GEOMYID^E THE DIPODID.E THE THERIDOSIYID.E THE OCTODONTID.E THE HYSTRICID.E THE CHIN- 

 CHILLIDJE THE DASYPROCTID.E THE CAVIID.E THE LEPORID^E THE LAGOMYID.E Mesotherium o-istatum 

 Difficulties concerning it Mr. Alston's Suggestion THE HEBETIDENTATA Teeth Skull Skeleton Conclusions 

 regarding it Table of Rodent Families Concluding Remarks. 



THE majority of the preceding families are more or less clearly represented by fossil remains, 

 either in the younger strata of the earth's crust, or in those cave-deposits of comparatively recent date 

 which have furnished so many interesting relics of the Mammals of former days. It must be 

 remarked, however, that while a considerable number of fossil Rodents have been named and 

 described by palaeontologists, the materials upon which many of them have been established are very 

 imperfect ; in a great number of cases isolated molar or even incisor teeth furnish the sole evidence 

 of the existence of creatures which were manifestly Rodents, but of which the other characters are 

 rather difficult to divine from such scanty material. Still, imperfect as may be " the record of the 

 rocks " in this as in other instances, it is in some parts sufficiently complete to enable vis to trace 

 back the existence of many forms of gnawing Mammals through a long period of geological time. 



Of the SCIURIDJE a considerable number of fossil species have been recorded. Species of the genera 

 iSciiirus, Arctomys, and Spermophilus, some of them identical with those still existing, have left their 

 remains in Post-Tertiary deposits and in bone-caves in various parts of Europe ; while species be- 

 longing to the first two genera, and to the American genus Tamias, have been detected in similar 

 situations in North America. A few forms referred to the same genera go down still lower in the 

 series of geological formations. True Squirrels are recorded from Miocene and Upper Eocene deposits 

 in France, and a single species from the Tertiaries (probably Miocene) of Colorado ; Marmots from 

 Pliocene and Miocene beds in the South of France, and from a Pliocene deposit in Nebraska ; and 

 a Spermophile from the Miocene of Weisenau in Germany. 



Besides these examples of known types, several fossils have been obtained both in Europe and 

 America, which are regarded as indicating genera distinct from any now living. Plesiarctomys Gervaisii 

 is founded on a fragment of jaw with four molars, obtained from Upper Eocene beds near Apt, 

 Vaucluse. In its characters it appears to be intermediate between Squirrels and Mannots. Pseudo- 

 sciurus suevicus, from the Upper Eocene (Bohnerz) of Wiirtemberg, seems to differ from all other 

 Sciuridte in the form of the molar teeth of the lower jaw, which are somewhat elongated, and 

 have four tubercles arranged in two pairs, each pair being connected by a ridge. From the Tertiary 

 deposits of the western territories of the United States, Professors Cope, Marsh, and Leidy have 

 described several Sciurine Rodents as belonging to genera now extinct : thus Paramys has five 

 species ; Sciuravus (perhaps identical with the preceding), three ; Heliscomys, Myaops, Colonymys 

 Taxyrnijs, and Tillomys, one or two species each. 



Of the ANOMALUsnXA and HAPLODOXTiD^E no fossil remains are known. On the other hand, 

 a North American fossil Rodent, described by Dr. Leidy under the name of Ischyromys typus, 

 is regarded by Mr. Alston as the type of a distinct family, the ISCHYROMYID.E, nearly allied 

 to the Sciuridse, but also showing an affinity to the Beaver in some of its characters. The 

 specimen described and figured by Dr. Leidy was obtained by Dr. Hayden from Miocene deposits 

 in the " Bad Lands " of Wyoming. It was originally referred to the Sciuridse, with which it agrees 

 in its dentition, but is distinguished by its large infra-orbital opening, the presence of a sagittal crest, 

 and the absence of post-orbital processes. The parietal region of the skull is much narrowed, and 

 in this, as in the large size of the infra-orbital opening, Ischyromys resembles the Musk Rat. 



Two other forms must be referred to here. Under the name of Pseudotomus hians, Professor Cope 

 has described the remains of an animal which he believed to have been about the size of an Agouti, 

 and originally thought to belong to the order Edentata. Subsequently he referred it to the Sciuridse ; 

 but both Mr. Alston and Mr. Allen think that it may belong to the family Ischyromyidae. In some 



