187).] Extinct American Rhinocercses and their Allies. 7716 
II. ? Digits. 
Incisors ? ; canine 2; posttympanic bone distinct; an osseous tuberosity on each 
si tiara Vindale Ue eaeee cass 6 Diceratherium. 
III. Three anterior digits. 
Incisors “$1; canines 2; no horn; posttympanic bone distinct....._ Aphelops 
Incisors 4; canines 9; a dermal eis poniymparie distinct... Ceratorhinus. 
Incisors $; canines 2; a dermal horn; posttympanic alabis 
Incisors +; canines 2; a dermal horn; posttympanic process coössified with 
postglenoid "n NO Marea Septal... ineo c subisis imocerus. 
Incisors 2; canines ?; a dermal hom; PERE process not united with 
postglenoid; no nareal osseous septu Atelodus 
Incisors %2; canines ®; a dermal horn; ye ne coéssified with post- 
id; an osseous septum dart Caelodonta 
My catalogue of species of the ab enera contai ty-eight 
names, of which six belong to living species: The latter are Cera- 
torhinus sumatranus Cuv., C. lasiotis Scl; Rhinocerus unicornis L. ; 
and R. sondaicus Cuv., all from Asia and Malaysia; and Azelodus 
bicornis L. and A. simus Burch., of Africa. It is possible that a spe- 
cies of Aphelops still exists in some of the Indian islands, in the 
Rhinocerus inermis Less. There are probably several distinct fos- 
sil species not in the list; but their characters have not yet been 
sufficiently made known to enable me to refer them to their 
proper places. It will be observed that eight species have been 
found in North American formations, ten in European, and three 
in those of Hindostan. It appears also that no extinct species of 
the true genus of Rhinocerus has yet been found in North Amer- 
ica or Europe, and that no extinct rhinoceros of North America 
which is known, possessed a median dermal horn. 
It can readily be seen that the genera above defined form a grad- 
uated series, the steps of which are measured principally by suc- 
cessive modifications of four different parts of the skeleton. 
These are, first, the reduction of the number of the toes of the 
anterior foot; second, the reduction in the number and develop- 
ment of the canine and incisor teeth; third, the degree of closure of 
the meatus auditorius externus below ; and fourth, in the develop- 
ment of the dermal horns of the nose and its supports. While 
these characters have the tangible and measurable quantities which 
render them available for generic diagnosis, there are others which 
possess a similar significance, and which I now notice, so far as 
they are observable in the extinct species of North America. 
I premise by observing that the Aceratheria and Diceratheria of 
this continent have only | been found in the eastern and western 
divisions of the White River formation, while the species of 
Aphelops are confined, so far as is known, to the Upper Miopa 
ae Loup River formation. 3 
