52 F. B. Loom is — Rhinocerotidce of the Lower Miocene. 



earlier forms from the John Day beds showing but little com- 

 plication ; but with the advance of time the crochet and 

 crista develop, and increase progressively until they meet and 

 isolate the median fossette. The premolars of any jaw grade 

 in their characters unto the molars, but there is a tendency for 

 the premolar to attain any feature earlier than the molar. 

 The upper canine is wanting but that of the lower jaw is 

 moderately developed, having a triangular cross section. The 

 incisors are reduced to {, the upper one being elongated and 

 oval in section as in Aceratherium, while the lower incisor is a 

 mere button-like rudiment. The first lower premolar is 

 usually wanting ; so that the generic dental formula is | f i f • 

 It is in the John Day beds of Oregon that the first Dicer- 

 atheres are found, full fledged as to the nasal horn cores ; but, 

 were only the dental series of such a form as D. armatum con- 

 sidered, the simple cross ridges and well developed cingulum, 

 would proclaim it an Acerathere. On the other side the spe- 

 cies Aceratherium tridactylum from the Protoceras beds has 

 a pair of low roughened bosses on either nasal bone seeming 

 to indicate incipient horns. This species was considered by 

 Hatcher* as already a Dicerathere, but Osborn places it among 

 the Aceratheres, where the writer would leave it, as the form 

 still has the second upper incisor and the more clolicocephalic 

 type of skull which characterizes the Aceratheres. However, 

 it is, as Osborn indicates, closely related and probably ancestral 

 to the Diceratheres, the White River Aceratheria being the 

 stock from which the genus Diceratherium arose. The John 

 Day species (especially D. armatuiru) are very Acerathere-like, 

 in the strong development of the cingulum and the absence 

 or weak development of the crochet and crista. The Euro- 

 pean species are likewise among the less specialized members 

 of the genus ; but they are differentiated by the strongly pro- 

 jecting protoconule fold, which has sometimes been described 

 as an "antecrochet." It seems to the writer simply an enlarge- 

 ment of the protoconule region, and is characteristic of both 

 D. minutum (=crozieri) and D. douvillei, and also of the 

 American species D. hesjoerium ; so that these three species 

 make a convenient and related sub-group. The later American 

 forms from the Lower Harrison beds all have crochet well 

 developed, D. niobrarense being the simplest of them, and 

 having an aspect wery suggestive of the John Day phase. The 

 other species have a crista, which the crochet tends to meet. 

 On the wall of the crochet away from the median fossette are 

 often tiny ridges which give the enamel a characteristic wavy 

 appearance. The latest species known is D. oregonensis, in 

 which the crochet and crista are broadly united. 



* Amer. Geologist, vol. xx, p. 313, 1897. 



