Tertiary Vertebrata of the West. 515 



morpha in the highest Miocene. There is, moreover, an interesting 

 affinity between the extinct and living genera, in the circumstance 

 that all the ancient genera differ from their modern representatives in 

 the greater constriction of the skull behind the orbits, acccompanied 

 by an absence of postorbital processes. The genus Schirus is known 

 from three species, two of which are about the size of the grey and 

 red Squirrels of North America. Gymnoptychiis appears to be a 

 Squirrel nearly related to Sciurus, but differs in having the tubercles 

 and crests of the molar teeth more complex than in any existing 

 genus, except Pteromys. There are two species which differ in size 

 and in proportions of the lower molar teeth. 



Meniscomys is another type in which the dentition resembles that 

 of Pteromys, but differs in the upper molars wanting the re-entering 

 inflexion of the enamel. Four species are described, which differ in 

 the length of the roots of the molars, in the character of the dental 

 crests, and the plications of the inferior molar teeth. 



It is observed that there is a suggestive resemblance between 

 Meniscomys Mppodus and Haplodontia rufa, now living in Oregon, 

 which is indicative of a common origin. This ends the history of 

 the true Squirrels. 



Ischyromys is the type of a distinct family ; for, though it resembles 

 the Squirrels in dental characters, the infraorbital foramen occupies 

 the position, at the origin of the zygomatic arch, seen in Porcupines 

 and Cavies. The superciliary ridge and postorbital process, seen in 

 most Squirrels, is lost ; and there is a contraction between the orbits, 

 which approximates towards the Beaver. Beavers are represented by 

 two species of the genus Castor. C. peninsulatus is about the size of 

 the large prairie Marmot, and is abundant in the Miocene rocks of 

 Oregon. G. gradatus is somewhat smaller. Heliscomys vetus is the 

 smallest Mammal of the White Biver beds, being about the size of 

 Mas musculus. Its affinities are open to some doubt ; the four 

 inferior molars have crowns with four cusps in two transverse 

 pairs and a broad ledge on the external side of the cusps. 



The Myomorpha are represented by two families, Muridaa and 

 Geomyidaa. Eumys elegans, of Leidy, is a Bat found in the White 

 Biver rocks in Eastern Colorado, previously known from Dakota. 

 The molar teeth are as large as those of the Norway Bat, but the 

 muzzle only two-thirds as long, so that the animal was smaller and 

 more robust. Hesperomys nematodon is a representative of an exist- 

 ing genus, and indicates a Bat as large as the red Squirrel. The 

 genus Pacicidus, which has three superior molars, differs from Sig- 

 rnodon and Neotoma, in having three external inflexions of the enamel, 

 instead of two, in the upper molars. Entoptyclms, known from five 

 species, is referred to the Georayidae. The molar teeth are four above 

 and four below, rootless, with prismatic crowns, which differ from 

 those of Perognathus, in having the enamel loop cut off and isolated. 

 The skull wants the vacuities and large foramina seen in many 

 Bodents. The bones of the skeleton in general resemble those of the 

 genus Tlwmomys. The species are defined by the characters of the 

 superciliary borders, the length of the skull, and width of the pre- 



