484 GEOLOGICAL SURVEY y OF THE TEREITOKIES. 



both iu Dakota and Colorado, and it is a point worth investigating 

 whether they were really derived from beds of that horizon. The su- 

 perior size and the color and mineral character of these specimens 

 refer them as probably parts of the same species at least. The sym- 

 physis contains the alveoli for, and basal portions of roots of, one pre- 

 molar, (with two roots,) one canine, and two incisors, on each side. The 

 incisors were evidently well developed, and indicate in the clearest 

 manner that Titanotherimn and Si/mhorodon are distinct genera. With 

 a considerable number of mandibles at my disposal, I have failed to- find 

 any trace of inferior incisors in the latter genus ; and if they were pre- 

 sent at any time, it must have been only during early youth, and as a 

 part of the deciduous dentition. 



Prof. O.C. Marsh has recently* described, under the name of Bronto- 

 tJierium^a, species allied to Titanotheriwn ijroutii, in which the mandibular 

 dentition is stated to be I., 2; 0., 1; P.m. ,3; M.,3. Ifthe mandibular frag- 

 ment described by Professor Leidy as presenting four premolars belongs 

 truly to the T. proutii^ then the form described by Professor Marsh will 

 occupy a position between it and Symborodon. Finally, Dr. Leidy de- 

 scribed the horned snout of a species of this group under the name of 

 Megacerops coloradensisA The specimen differs in various respects 

 from any species observed by me in Northern Colorado. It was derived 

 from a locality remote from that which contains the Symhoroda, from 

 a bed of coarse sandstone entirely different in mineral characters from 

 the argillo-calcareous beds from which the fossils described in this re- 

 port were obtaihed. In the absence of knowledge as to the dental char- 

 acters of this animal, and the consequent uncertainty as to which of 

 the three genera above named it belongs to, I leave it for the present. 



Six well-defined species of the genus are known to the writer, which 

 vary in dimensions from that of the Indian rhinoceros to nearlj'- the 

 size of the elephant. They may be readily recognized by the following 

 characters ; the most important of which is the basal cingulum of the 

 premolars : 



A. Horn above the preorbital region : 



I. Premolars without inner basal cingulum : 



a. Nasal surface continuous with front : 



Horn-cores large, compressed; zygomata enormously ex- 

 ^ panded ; cranium depressed. — S. bucco. 



Horn-cores large ; zygomata not expanded ; cranium rather 



elevated ; nasals very short. — 8. altirostris. 

 Horn-cores mere tubercles; nasals not shortened; — S.lielo- 

 ceras. 



II. Premolars unknown ; nasal plane sloping downward from a 

 roof-shaped angle with the frontals : 



Horn-cores very elongate, subcylindric, curved, partly com- 

 posed of maxillary bones at base. — F. acer. 



III. Premolars with a strong internal basal cingulum : 



Horn-cores short, very stout, and subtriangular in section ; 

 nasal bones more elongate. — S. trigonoceras. 



Horn-cores small, compressed, followed by a tuberosity 

 on the frontal bone. — S. hypoceras. 

 AA. Horn-cores above the orbit : 



Premolars with internal cingulum ; horn-cores stout, com- 

 pressed; nasals longer. — 8. ophryas. 



* Amer. Journ. Sci. Arts, 1873, )?. 486. 



I Report (4to) on Geol. Survey Territories, p. 239, pi. I. figs. 2, 3, and II, fig. 2. 



