484 GEOLOGICAL SURVEY/ OF THE TERRITORIES. 



botli 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 Titanotherium and Symhorodon 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. 0. Marsh has recently* described, under the name of Bronto- 

 f/^ermm, a species allied to Titanotlierium proutii, in which the mandibular 

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

 ment described by Professor Leidy as presenting four premolars belongs 

 truly to the T. proutii, then the form described by Professor Marsh wdl 

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

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

 3Iegacerops coloradensis.f 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 obtaitied. 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 knowu to the writer, which 

 vary in dimensions from that of the Indian rhinoceros to nearly 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. hucco. 

 ' Horn-cores large ; zygomata not expanded ; cranium rather 



elevated ; nasals very short. — 8. altirostris. 

 Horn-cores mere tubercles; nasals not shortened. — 8. lielo- 

 ceras. 



II. Premolars unknown ; nasal plane sloping downward from a 

 roof-shaped angle with the frontals : 



Horn-cores very elongate, subcyliudric, 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.— >Sf. trigonoceras. 



Horn-cores small, compressed, followed by a tuberosity 

 on the frontal bone. — S. Jiypoceras. 

 AA. Horn-cores above the orbit : 



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

 pressed ; nasals longer. — 8. opliryas. 



* Amer. Journ. Sci. Arts, 1873, p. 486. -, tt ^ r. 



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



