Troxell — Rodents of Genus Ischyromys. 123 



Aet. VIII — Oligocene Rodents of the Genus Ischyromys; 

 by Edwaed L. Teoxell. 



[Contributions from the Othniel Charles Marsh Publication Fund, Peabody 

 Museum, Yale University, New Haven, Conn.] 



A great diversity of form is shown in the rodents of the 

 Early Tertiary period; some of these have lived even to 

 the present time, others became extinct long ago and are 

 only remotely connected with existing genera. 



Leidy in 1856 1 first described the squirrel-like rodent 

 which he called Ischyromys typus, and in 1889 2 published 

 a detailed morphology of the skull and teeth, having not 

 only a full appreciation of the relationship to the modern 

 Schirus but also a realization that the one was not 

 ancestral to the other. 



Cope in 1873 3 and 1881 4 added new points of interest 

 concerning the skeleton, and further emphasized the rela- 

 tionship of this genus to the squirrels in the nature of the 

 teeth, but to other genera, Arctomys, Castor, etc., in skull 

 and skeletal features, and he agreed with Alston that 

 there should be a separate family, the Xschyromyidse. 

 Several new genera and species were made by Cope, 

 which he afterward concluded were synonyms of Ischy- 

 romys typus Leidy or belonged to entirely different 

 groups. 5 Colotaxis cristatus and Gymnoptychus chry- 

 sodon are equivalent to Leidy's genus and species; they 

 are based on what one finds to be moderate-sized speci- 

 mens with or without the variable tubercles between the 

 lobes of the lower molars. 



Matthew 6 has worked out the interrelation of the early 

 rodents and discusses in full the family Ischyromyidse. 

 His new species and subgenus Ischyromys (Titanothe- 

 riomys) veterior was based on the narrow heel of M 3 , the 

 narrow incisors, and the earlier geological age, i. e. Titan- 

 otherium beds. He places the following genera under 

 Ischyromyidae Alston : Ischyromys, Paramys, Sciuravus, 



1 Joseph Leidy, Proc. Acad. Nat. Sci. Phila., 8, 89. 



2 Joseph Leidv, Jour. Acad. Nat. Sci. Phila. (2), 7, 335, pi. 26, figs. 1-6. 



3 E. D. Cope, Pal. Bull. No. 15, p. 1 ; ibid., No. 16, p. 5. 



4 E. D. Cope, Bull. U. S. Geol. and Geog. Survey Terr., vol. 6, 366-368. 



5 See O. P. Hay, Science (2), 10, p. 253, 1899. 



6 W. D. Matthew, Bull. Amer. Mus. Nat. Hist., vol. 28, 43-72. 



