404 MEMOIRS OF THE CARNEGIE MUSEUM. 



while according to recent work by European authors Prohyracodon orientale Koch 



is regarded as the earUest and most primitive representative of the Rhinocerotidse.*^ 



Diceratherium armatum of the John Day formation has the dentition as well 



as certain other features of the skull in a much less advanced stage of development 



2 3 



Fig. 2. Diceratherium minutum (Cuvier). M- X i. After Cuvier. 

 Fig. 3. Diceratherium douvillei. M^ X i. After Osborn. 



than D. annectens of the same deposit (Compare PI. LVII with text-figure 11, 

 also with PL LXIII, fig. 6, and PI. LXVI, fig. 1). In the latter form we naturally 

 might expect to meet with a greater range of anatomical variations, especially in 

 connection with the dentition. We may reasonably expect to find grinding teeth, 

 having crests ranging from those which are quite plain to those which have the 

 various incipient projections, as crista, crochet, anti-crochet, etc. It is far from 

 my mind to depreciate some, or all, of these characters; on the contrary, indeed, 

 it is reasonable to expect that the dentition should be one of the first parts of the 

 organism to undergo modification with a change in the environment. It is never- 

 theless questionable whether the absence or presence of a crista, a crochet, and 

 anti-crochet, more or less developed, or of a cingulum of greater or less prominence, 

 should constitute a valid specific character in Diceratherium. I very much doubt 

 whether these characters are of sufficient constancy to be relied upon to establish 

 specific distinctions in a large collection of individuals from a given locality. Stress 

 has in times past been laid upon the development of branches or spurs of different 

 lobes of the cheek-teeth. It is plainly evident that D. annectens, as the result of its 

 mode of life, was already in the time of the John Day more advanced, having filled 

 out the grinding surface of its teeth more than its contemporary, D. armatum. In 

 animals representing a later development in geological time, we should expect 

 to find similar evidence of progression, and in a large assemblage of individuals 

 that not all the specimens, say of D. cooki for example, are provided with crista 

 and crochet united on the premolars and with crista small and crochet larger on the 

 molars, but that these features, being in a plastic stage of development, would he 

 found in an endless number of combinations from those less developed to those having 

 more complex forms, and all within one species. 



« Abel, O., I.e., p. 24, 44-4.5, 49. 



