Troxell — Rodents of Genus Ischyromys. 12£ 



Y. P. M.) is about the same length but broader, especially 

 across the temporal portion of the zygomatic arch ; there 

 are strong post orbital processes which are entirely absent 

 in the fossil. Further distinctions of Cynomys are the 

 narrower antero-posterior dimension of the teeth, the 

 large P 3 and MJ-, the anterior position of the infraorbital 

 foramen resulting from the large groove anterior to and 

 beneath the zygomatic projection of the maxillary, the 

 large angle of the lower jaw with numerous additional 

 projections for muscle insertions, the decrease in the size 

 of the coronoid, the longer segments of the hind limbs 

 with the cnemial crest of the tibia and the third trochan- 

 ter of the femur far up on the shaft, the single groove 

 (instead of double as in the fossil) on the distal end of the 

 tibia for the tendon of the deep digital flexor running to 

 the sole of the foot, and the large, longer claws. 



Important similarities, on the other hand, may be noted 

 in the structure of the teeth with the cross crests of 

 enamel, the shape of the occipital region of the skull and 

 the sagittal crest, the form of the nasals and premaxil- 

 laries, the peculiar supra-auditory foramen, and the same 

 dental formula. There is a remarkable agreement in the 

 size and form of the humerus and of the distal end of the 

 femur in each genus, both being distinct from those of the 

 squirrel, Sciunis. In the humerus there is a strong del- 

 toid crest, a wide condyloid crest on the outer side, and a 

 supracondylar foramen on the inner side distally. This 

 foramen, according to Flower, is a departure from the 

 general rodent humerus, but is seen in the wombat and in 

 certain carnivores. It is thought probable that the spiral 

 condylar grooves on the humerus permit a freer move- 

 ment of the ulna and allow its rotation for swimming or 

 burrowing as in the beaver, while the large deltoid prom- 

 inence and the lateral condyloid crest, again like those of 

 the beaver, tell of the strong muscles which manipulated 

 the front limb. 



An objection which may be offered to any theory 

 attempting to derive Cynomys from Iscliyromys is the 

 presence of the very small third premolar in the latter. 

 This resolves itself into the question whether P 3 of the 

 fossil is reduced to a vestigial state and therefore much 

 advanced in evolution, or whether it is small and yet unde- 

 veloped but has within it the potential molariform tooth 

 of Cynomys. 



