G44. MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
SCIURAVUS NITIDUS Marsh. | See anted, p. 935. 
SCIURAVUS UNDANS Marsh. See anted, p. 935. 
SCIURAVUS PARVIDENS Marsh. See anted, p. 936. 
? SCIURAVUS —— Leidy. See anted, p. 936. 
Incerte sedis. 
HELISCOMYS VETUS Cope. See antcd, p. 936. 
MYSOPS FRATERNUS Leidy. See anted, p. 937. 
MYSOPS MINUTUS Leidy. See anted, p. 937. 
COLONYMYS CELER Marsh. See anted, p. 938. 
TAXYMYS LUCARIS Marsh. See anted, p. 938. 
TILLOMYS SENEX Marsh. See anted, p. 938. 
TILLOMYS PARVUS Marsh. See anted, p. 939. 
CASTORIDA. 
EUCASTOR TORTUS Leidy. See anted, p. 451. 
STENEOFIBER NEBRASCENSIS Leidy. See anted, p. 453. 
STENEOFIBER PANSUS Cope. See anted, p. 455. 
ISCHYROMYIDZi. 
The family Ischyromyide was recently proposed by Mr. E. R. Alston* for the recep- 
tion of Dr. Leidy’s genus Ischyromys. Mr. Alston’s diagnosis of the group, based on 
Dr. Leidy’s figures and descriptions of his Ishchyromys typus, is as tollows :—‘ Dentition 
as in Seiuride. Skull resembling Castoride, but with the infraorbital opening large, 
a sagittal crest, no postorbital processes, palate broad, basioccipital keeled ”.t 
Genus ISCHYROMYS Leidy. 
Ischyromys Lewy, Proc. Acad. Nat. Sci. Phila. 1856, 89. 
ISCHYROMYS TYPUS Leidy. 
Ischyromys typus Lerpy, Proce. Acad. Nat. Sci. Phila. 1856, 89; 1857, 89; Journ. Acad. Nat. Sci. Phila. 2d 
ser. Vil, 1869, 335, 405, pl. xxvi, figs. 1-6. - 
About the size of a Muskrat (Fiber zibethicus). Form of the skull somewhat 
resembling that of the Beaver (Castor fiber), but the interparietal region is greatly 
narrowed, more even than in the Muskrat, with the frontal relatively wider. The infra- 
orbital opening was apparently as large as in the Muskrat. Upper grinding teeth five, 
‘constructed after the plan of those of the Squirrel family”. 
The lower grinders, four in number, also resemble those of the Sciwride, and the 
lower-jaw fragments are described as resembling the corresponding portions of the jaw 
of a Squirrel; “but the impression of the masseter is comparatively feeble, and only 
reaches as far forward as the position of the back part of the second molar tooth”. 
* Proc. Zool. Soc. Lond. 1876, 78. 
t Mr. Alston adds in a foot-note :—“ It seems probable that Pseudomys Cope [ = Pseudotomus Cope} 
(Proc. Amer. Phil. Soc. 1872, 467), from ihe Eccene of North America, may belong to this family.” 
(ALsTON, I. ¢.) 
