SCIUEOMORPHA. 885 



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

 in the HystricidcB, and by the position of the angular apophysis of the man- 

 dible 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 poi-tion, 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. R. Alston 

 for the reception of this genus, to which he thinks with mef Plesiardomys 

 {■=zPseudofo)»us) should be referred. He thus defines the family:! "Den- 

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

 opening large, a sagittal crest ; no postorbital processes ; palate broad ; 

 basioccipital keeled." 



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

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

 as follows : 



Dentition as in Scmridce; 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 laminae are characters found in the Muridoe. 



But one species of this genus is known. 



Ischyromys typus Leidj^ 



Proceedings Academy Philada. 1856, p. 89. Extinct Fauna of Dakota and Nebraska, 1869, p. 335. Cope 

 Annual Report U. S. Geol. Surv. Terrs. 1873 (1874), p. 477. Colotaxh criatatus Cope Paleontolog- 

 ieal Bulletin No. 15, p. 1. Gymnoptychus ckrysodon Cope, Paleontological Bulletin No. 16, p. 5. 



Plate LXVII ; figs. Ul-2. 



The principal characters of this species have been given by Dr. Leidy 

 «s above cited. I i-epeat the most important of these, and several which 

 have not been previously noticed. 



In the molar teeth of both jaws there is no transverse crest arising 

 from the connection between the adjacent horns of the crescents. The 



' Catalogue Method, et Descr. de Vertebras Foss. de le Bass, de la Loire, 1853, p. 32. 

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

 ; Proceed. Zool. Society London, 1676, p. 78. 



