Peterson: A Group of Stenomysins. 307 



specialized than recent forms. Their long-crowned molars seem to 

 indicate that the creatures had continued to feed for many general i> ms 

 on hard grasses on open prairies throughout long geologic times, and 

 that thus was broughl about a development decidedly more advani ed 

 than is found in any of the recent forms. Many other features ot 

 the skull are also entirely different. The shorter humerus and longer 

 radius and ulna are marked advances made by the fossil forms, while 

 the phalanges are shorter, and there is much less indication of the 

 tylopod pads than in the llama. The limbs were even slenderer than 

 in the llama and the heavy muscles of the limbs were placed close to 

 the body. There were undoubtedly short, blunt hoofs, and possibly 

 incipient cushions, though in a much less developed state than in 

 the recent tylopods, as stated above. 



In previous publications it has been stated that the skeletons of 

 these small animals were found by the dozens imbedded in a Miocene 

 sediment of packed sand in Sioux County, Nebraska (see Ann. Car. 

 Mrs., Vol VII, 1 9 1 1 , pi. XI J). That they were gregarious in habit, 

 there can be very little doubt. It requires only a slight stretch of 

 imagination to picture a herd of Stenomylins, pursued by some car- 

 nivore (Daphcenodon), taking to a stream of water and attempting 

 crossing to the opposite shore, but finding the current too swift, 

 being taken ruthlessly along, never again to reach the land alive. The 

 deposition of these skeletons of all ages and sexes, sometimes found in 

 most perfect preservation, appears to agree best with the idea that 

 they were laid down in a bend, or against a sandbar of a stream and 

 were covered up very rapidly. They might possibly have been cov- 

 ered up by rapidly moving sand on land. 



The cause of the extinction of this fleet-footed grazing quadruped 

 is little short of an enigma. When we compare its osteology with 

 the living forms we are obliged to admit that the skeletons of Ste- 

 nomylus show us a form in many respects better equipped for life 

 in an open country than the forms which occupy the plains at the 

 present time. Paleontological evidence tends to lead us to regard 

 the evolution of grazing types as far more favorable to continued ex- 

 istence than browsing types. The latter were more or less hindered 

 through the gradual changes of the flora from the softer and more 

 succulent vegetation in the earlier Tertiary to the harder grasses in 

 the late Miocene and Pliocene times. What then was the cause of 

 tin extinction of Stenomylus, an animal already so specialized as 



