26 



THE MERYCOIDODONTID/E 



stock. Eomeryx and Agriotherium are considered synonyms of Protoreodon. Hyomeryx is also 

 closely related but not in the main line of descent. 



The lower Oligocene has produced three genera, Bathygenys, Oreonetes, and Limnenetes, 

 about which we know all too little. The material of Bathygenys is so fragmentary that it war- 

 rants no definite conclusions as to its position, but at present it seems to be a side branch, closest to 

 Limnenetes. 



Scott has long maintained that the presence of inflated bullae is a primitive condition and that 

 the development of small bulls: is a secondary development. Van der Klaauw (1931) has also 

 shown this to be true, but in a somewhat different sense. The Uinta forms show the small bullae, 

 together with Oreonetes and some species of Merycoidodon, and these have been put in a separate 

 phylum by Loomis ( 1 924) and earlier by Douglass. The size of the bulla in proportion to the size 

 of the skull varies greatly in the different genera. While it is a fact that the inflated bulla is gener- 



Protoreodon 

 medius 



Oreonetes 

 anceps 



Merycoidodon 

 culbertsoni 



Pm.4 Pm.3 

 Lower 



Pm.4 Pm.3 

 Upper 



F, c- 3. — Third and fourth premolars of Protoreodon, Oreonetes, and Merycoidodon. (After Loomis, 1924.) 



ally present in all forms above the middle Oligocene, yet it is proportionally as small in relation to 

 size of skull in some of these genera as it is in Merycoidodon culbertsonii. In Merycoidodon 

 gracilis, a true member of this genus, the bulla is relatively as large as in Eporeodon, and it is highly 

 probable that Leidy's "Oreodon bullatus" should be placed with Merycoidodon. 



In Agriochcerus antiquus dakotensis Thorpe (1921D) the holotype shows the condition of the 

 bulla on one side to be very much like that in Oreonetes and Merycoidodon culbertsonii, while 

 the other side shows a large inflated bulla. This seems to indicate that the inflated one on the first 



This question of the bulla is one which I have long considered in connection with Eucrotaphus 

 and Eporeodon. Is it not possible that some of the species which have been assigned to these two 

 genera should really be referred to Merycoidodon and that the latter genus extended into the upper 

 Oligocene and perhaps into the lower Miocene? There are several examples in this family of the 

 ancestral stock persisting side by side with the presumed derivatives. The true Eporeodon shows an 

 advance in tooth and skeletal structure over Merycoidodon, but there are several borderline species 

 in which the chief and almost the only deciding factor is the presence of large bullae. If by analogy 

 with M. gracilis other species of Merycoidodon had bulls larger than those of M. culbertsonii, the 

 two genera would be equalized to a much greater extent than at present, and the great expansion in 



