THE MAMMALIA OF THE DEEP RIVER BEDS. 137 



away towards the ends. (4) The unusual development of the spine of the scapula 

 is an indication that the clavicle had not been entirely lost. (5) Wincza's observa- 

 tion (]STo. 36) that a transitory rudiment of a bony clavicle is developed in the sheep, 

 points to the conclusion that this element has not been eliminated from the artiodac- 

 tyls for so long a period as has been generally supposed. (6) Admitting that the 

 structure under discussion really represents the clavicle, its very small size and loose 

 attachments (for it is in contact with neither the scapula nor the sternum) will ex- 

 plain why it has not yet been observed in the more ancient forms of ungulates. Only 

 by the rarest chance could such a bone be preserved in its natural position. 



Unless, therefore, we are prepared to assume that a single bone of some small 

 animal has become accidentally entangled with this skeleton of Mesoreodon, and in 

 such a way as to exactly simulate the position of the collar bone, which is certainly 

 highly improbable, there would seem to be no escape from the conclusion that the 

 clavicle was present in this genus. However, other specimens will be required before 

 we can be entirely satisfied on this point. 



In this connection it may be noted that the simpler and less developed scapular 

 spine of Oreodon would lead us to infer the absence of a clavicle in that genus. But, 

 assuming this to be the case, we cannot yet determine the significance of the fact since 

 so little is known of the skeleton of those White Kiver species in which the tympanic 

 bullae are inflated and which are presumably the ancestors of Eporeodon. In the 

 absence of knowledge on this point we cannot tell whether the supposed clavicle of 

 Mesoreodon should be regarded as a persistent rudiment or as a case of reversion and 

 the reacquisition of a lost structure. The former alternative would certainly seem 

 to be more probable, and, if it is true, it may serve to explain the very general differ- 

 ence between artiodactyls and perissodactyls with regard to the development of the 

 acromion. As is well known, this structure is in nearly all artiodactyls large and 

 prominent, while in even the Eocene perissodactyls the acromion is absent. If we 

 may assume that the clavicle persisted longer in the former group than in the latter, 

 this difference would be accounted for. 



The humerus (PI. IV, Fig. 34; PI. V, Fig. 37) is in general very similar to that 

 of Eporeodon, but has a decidedly stouter shaft ; the head is more convex and presents 

 much more posteriorly, less exclusively in the proximal direction. The external 

 tuberosity is of a different shape, its extremities being less produced as overhanging 

 hooks ; the internal tuberosity is also less developed, and in consequence the bicipital 

 groove is not so deep. The length of the shaft is about the same as in Eporeodon, 

 but the diameter, both transversely and antero-posteriorly, is much greater. The 

 distal end is of the shape which is characteristic and constant throughout the family. 



