COPS.J PALEONTOLOGY ^MIOCENE PERIOD. 477 



ISOHYEOMYS, Leidy. 



Proceed. Acad. Nat. Sci. Philad., 1856, p. 89.— Extinct Fauna Dakota and Nebraska. 

 335. — Colotaxis, Cope, Paleontological Bulletin, No. 15, p. 1. 



Char. gen. — The essential features are, deutition, I., i; C, %; M., f ; the 

 molars with two crescents on the inner side above, each of wliich 

 gives rise to a cross-ridge to the outer margin. In the mandibuhir series 

 the crests and crescents have a reversed relation. ISo cementum. 



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

 ridcB. 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, there 

 is a large foramen infraorbitale anterius, which occupies the elevated 

 position at the origin of the zygomatic arch seen in the Fiber, the porcu- 

 pines, and cavies. There is no superciliary ridge nor postorbital process 

 as in most Sciuridce, but the front is contracted between the orbits in 

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

 Fseudotomus, Cope. Both tlie last-named and Ischyromys present many 

 points of resemblance to PomePs tribe of Frotomyidw, but differ from 

 any of the genera he has included in it. 



ICHSYEOMYS TYPUS, Leidy, loG. cit. Colotaxis cristatus, Cope, Pal. 

 Bull., No. 15, p. 1. Gymnoptychus chrysodon, Cope, loc. cit., No. 16, p. 5. 



First upper molar a simple cone. Incisors quite compressed. First 

 inferior molar a broad oblong ; the cnsps opposite, the anterior close to- 

 gether. The two posterior cross-crests do not form a Y ; the anterior 

 being interrupted at the cusp. There is a delicate tubercle between the 

 outer cusps of the three last molars. The incisor is compressed ; the 

 anterior and outer faces being separated by an angle. 



Measurements. 



M. 



Length of molars 0.0140 



Length of penultimate molar 0033 



Width of penultimate molar 0035 



Width of first molar 0030 



Length of first molar 0035 



Depth of jaw at i)enultimate molar : 0090 



Depth of incisor-tooth 0040 



Width of incisor-tooth 0020 



The skull is broad and stout but not depressed ; muzzle broad above, 

 short ; front moderately contracted ; no postorbital processes. 



This species varies considerably in the form of the premolar teeth, 

 and I believe the above names refer to varieties, not to species, 



PAL^OLAGUS, Leidy. 



Proceed, Acad. Nat. Sci. Philad., 1856, p. 89. — Extinct Mamm. Dakota and Nebraska,,",. 

 331. 



As observed by Leidy, this genus presents the same number of teeth 

 as in the existing rabbits, viz, I., f ; 0., gj M., f ; and that the difference 

 consists in the fact that the first molar possesses two columns, while in 

 Lepus there are three. Having collected a great number of remains of 

 this genus, I am able to show that it is only in the immature state of 

 the first molar that it exhibits a double column, and that in the fully 

 adult animal it consists of a single column with a groove on its external 



