322 F. B. Loomis — Osteology and Affinities 



III and mt. IV being fused rather more than half their length. 

 From the foregoing it will appear that S. crassipes is a species 

 closely resembling 8. gracilis, but differentiated by the short- 

 ening of the limbs, especially below the elbow and knee, a 

 heavier shorter-limbed type being thus evolved. 



Affiiiities. — In his descriptions Peterson has placed the 

 genus Stenomylus among the Tylopoda, near the long-limbed 



Fig. 30. 



Fig. 30. Stenomylus crassipes, metatarsus and proximal phalanges, x 1/2. 



type Oxydactylus ; while Matthew* has associated the genus 

 with Hypisodus among the Hypertragulidse. In order to 

 show concisely the points of likeness and difference to one or 

 the other of these types, the characters of Stenomylus are in 

 the above diagram arranged in parallel columns with those 

 of Hypisodus on the one side and with Poebrotherinm on the 

 other, the latter being used as illustrating a primitive tylopod. 

 From the foregoing table it is evident that in the matter of 

 dentition ( number 1 - 5 ) the genus Stenomylus approximates 

 Hypisodus; but in the other skeletal features it is distinctly 

 allied to the Tylopoda. This is especially marked in such 

 deep-seated and fundamental characters as the cancellous bone 

 in the bulla, the position of the vertebrarterial canal, the matter 

 of the fusion of the magnum and trapezoid, the matter of 

 fusion of the navicular and cuboid, and the manner in which 

 the lateral digits have been reduced. These are characters 

 less liable to modification on a change of habit, while the den- 

 tition is the first to respond to changes in the matter of food. 

 I feel therefore that Stenomylus should be placed among the 

 Tylopoda. Then as it is evident that the dentition is aberrant 

 in its extreme hypsodont specialization, and presuming that 

 this characteristic has been acquired in conjunction with a spe- 

 cial feeding habit (which feeding habit would presumably be 

 the same as characterizes Hypisodus, but a parallel adaptation), 

 I feel that Stenomylus should be set off by itself. The habit, 

 which is general to forms having this hypsodont dentition, is 

 feeding on hard grasses, usually on the open prairies, the grass 

 having in its stem considerable quantities of silica which causes 

 extremely rapid wear of the teeth. I take it then that while 



* Bull. Amer. Museum Nat. Hist., xxiv, 1908, p. 539. 



