Q4 1 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



SCI1 RAVU8 NITIDUS Marsh. See anted, p. 935. 

 S( [UBAVUS CNDANS Marsh. See anted, p. 935. 

 SI n RAVI S PABVIDENS Marsh. See anted, p. 936. 



[UBAVUS Leidy. See anted, p. 936. 



Incertw sedis. 

 BELISCOMYS VETUS Cope. See anted, p. 936. 

 MYSI )PS FBATEENUS Leidy. See anted, p. 937. 

 MYSOPS M1NUTUS Leidy. See anted, p. 9:37. 

 COLONYMYS CELEB Marsh. See anted, p. 938. 

 TAXYMYS LUCARIS Maish. See anted, p. 938. 

 TILLOMYS SENEX Marsh. See an ted, p. 938. 

 TILLOMYS PAEVUS Marsh. See an ltd, p. 939. 



CASTORID^E. 



EUCASTOE TORTUS Leidy. See anted, p. 451. 

 8TENEOFJBEE NEBEASCENSIS Leidy. See anhd, p. 453. 

 STENEOFIBER PANSUS Cope. See anted, p. 455. 



ISCHYROMYID^E. 



The family Ischyromyidce 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 follows : — "Dentition 

 as in Sciuridce. Skull resembling Castor idee, but with the infraorbital opening large, 

 a sagittal crest, no postorbital processes, palate broad, basioccipital keeled ".t 



Genus ISCHYEOMYS Leidy. 



Ischyromys Leidy, Proo. Acad. Nat. Sci. Phila. 1856, 89. 



ISCHYROMYS TYPUS Leidy. 



Ischyromys typus Leidy, Proe. Acad. Nat. Sci. Phila. 1856, 89; 1857, 89; Journ. Acad. Nat. Sci. Phila. 2d 

 ser. vii, 1869, 335, 405, pi. 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 grindiug teeth five, 

 "constructed after the plan of those of the Squirrel family". 



The lower grinders, four in number, also resemble those of the Sciuridce, 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. Soo. Loud. 1876, 78. 



• Mr. Alston adds in a foot-note: — '• It seems probable that rseudomys Cope [ = Pseudotomus Copo] 

 (Proc Amer. Phil. Soo. 1872,467), from the Eocene of North America, may belong to this family.'' 

 (Alston, '. c.) 



