368 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.ri.- 



Dr. Leidy remarks that this genus belongs to the family of the Sciu- 

 ridce. This is indicated by the dental characters ; but in some other 

 respects there is a greater divergence from the squirrels and marmots 

 than is the case with the preceding genus, Gymnoptychus. Thus, the 

 large foramen infraorbitale anterius occui^ies the elevated position at 

 the origin of the zygomatic arch seen in the porcupines and cavies. 

 There is no superciliary ridge nor postorbital process as in most Sciu- 

 ridce, but the front is contracted between the orbits in the same manner 

 as, but to a less degree than, in Fiber, and the Eocene Plesiarctomys, 

 Brad. Both the last named and Ischyromys present many points of re- 

 semblance to Pomel's tribe of Protomyidce, but differ from any of the 

 genera he has included in it. 



This family is thus defined by Pomel: * "infraorbital foramen large as 

 in the Hystricidce, and by the position of the angular apophysis of the 

 mandible almost in the general plane of the horizontal ramus. The 

 jugal bone, at least in those species where we have observed it, is very 

 much enlarged at its anterior portion, and the orbit is almost superior." 



These characters apply to Ischyromys, excepting as regards the malar 

 bone, which is principally unknown in the latter. 



Another family, the Ischyromyidce, has been proposed by E. E. Alston 

 for the reception of this genus, to which he thinks with me t Plesiarctomys 

 (= Pseudotonms) should be referred. He thus defines the family : | " Den- 

 tition as in Sciuridce ; skull resembling Castoridce, but with the infra- 

 orbital opening large, a sagittal crest ; no j)ostorbital processes ; palate 

 broad ; basioccipital keeled." 



Doubtless Ischyromys belongs to an extinct family, but which of the 

 above names is available for it I do not yet know. I would character- 

 ize it as follows : 



Dentition as in Sciuridce; infraorbital foramen large, superior; ptery- 

 goid fossa large, with well-developed exterior as well as interior walls; 

 a sagittal crest. 



The superior position of the infraorbital foramen and the well-devel- 

 oped pterygoid laminse are characters found in the Muridce. 



But one species of this genus is known. 



CASTOE Linn. 



Syst. Nat. I, p. 78, 1766. — Steneofiber, E. Geoffr., Eevue Encyclop6dique, 1833. — "Chali- 

 comys Meyer, Neues Jalirbuch, 1838, p. 404, et 1846, p. 474." — Palceocastor Leidy, 

 Extinct Mammalia Dakota and Nebraska, 1869, p. 338. 



The family of the Castoridce differs from the Sciuridce in the absence 

 of postorbital angles or processes and the presence of a prolonged tube 

 of the meatus auditorius externus. In both of these points it agrees 

 with the Haplodontiidce, a family which Mr. Alston has distinguished 



* Catalogue Method, et Descr, de Vertebras Foss. dele Bass, de la Loire, 1853, p. 32. 

 t Annual Eeport U. S. Geol. Survey Terrs. 1873 (1874), p. 477. 

 t Proceed. Zool. Society London, 1876, p. 78. 



