238 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



between the canine and the first premolar; first premolar 

 small; third premolar becoming molariform; fourth premolar 

 molariform; symphysis long; symphyseal surface between 

 canines narrow; jaw contracted at the diastema; external 

 cingula moderately developed; internal cingula wanting; 

 mental foramen beneath the second premolar; coronoid process 

 short. 



Megacerops avus (Marsh), from the Oligocene of South 

 Dakota, has three pairs of inferior incisors but only three pre- 

 molars below on each side, and there is a short diastema behind 



Figure 202. — Type (holotype) skull of Bronlo- 

 therium hatcheri 



Top view. Nat. Mus. 1216. After Osborn. 1908. One-tenth 

 natural size. 



the lower canine. Its dimensions are greater than those of 

 M. primitivus. These two species are apparently the only ones 

 of the Oligocene titanotheres in which there are three pairs 

 of incisors in the lower jaw. 



In the Cypress Hills specimen the crowns of the incisors are 

 of a depressed spherical shape, with a tendency to come to a 

 rounded central point above. The second incisor is the largest, 

 and the first is slightly smaller than the third, which is the 

 most upright. The first is more procumbent than the second. 

 Between the inner pair is a very decided interval, leaving a 

 space of 6.5 millimeters between the crowns of the two teeth. 

 The crowns of the canines are broken ofi' (that of the right tooth 

 being restored in fig. 5 of pi. 6) and the right first premolar is 

 lost from its alveolus. * * * 



Keeping in mind the differences due to sex in titanotheres 

 generally and the apparent variability, both specific and indi- 

 vidual, of certain dental characters, such as the degree of devel- 

 opment of the cingula, the presence or absence of the first 

 premolar, the size of the canines, and the number of the incisors, 

 M. primitivus is apparently a well-marked species, characterized 

 principally, so far as known at present, by the breadth of the 

 mandible anteriorly (as compared with M. angustigenis) and 

 the presence of the fuU number of teeth, with a comparatively 

 long diastema behind the canines. 



This species, for which the name primitivus is used, is regarded 

 as representing a rather early stage in the development of the 



titanotheres. The general character of the dentition suggests 



the appropriateness of referring the species to the genus 



Megacerops. 



Measurements 



Millimeters 



Length of ramus 475 



Depth of same at posterior end of fourth premolar 74 



Depth of same at posterior end of second molar 81 



Depth of same from tip of coronoid process to lower 



border 247 



Maximum thickness of same beneath third molar 46 



Length of symphysis 144 



Distance apart of inside surface of base of canines" 31 



Length of premolar series 103 



Length of molar series 183 



Diameter of canines at base: 



Anteroposterior 18 



Transverse 16 



Diameter of second premolar: 



Anteroposterior 26 



Transverse 18 



Diameter of third premolar: 



Anteroposterior 32 



Transverse 23 



Diameter of fourth is premolar: 



Anteroposterior 35 



Transverse 27 



Etymology. — primitivus, primitive; in reference to 

 the presence of three lower incisors. 



Present determination. — The species is probably 

 valid. It is probably referable to Teleodus. 



Megacerops assiniboiensis nom. prov., Lambe, 1908 



Cf. Brontotherium curium, this monograph, page 574 



Original rejerence. — Contr. Canadian Paleontology, 

 vol. 3, pt. 4, pp. 51-53, pi. 5, fig. 6, 1908 CLambe, 

 1908.1). 



Figure 203. — Type (holotype) skull of Symborodon copei 

 After Osborn, 1908. Nat. Mus. 4711. One-tenth-naturai size. 



Type locality and geologic horizon. — Oligocene de- 

 posits of the Cypress Hills, Saskatchewan. Collection 

 of 1904. 



" In the mandible of M. angustigenis (No. II) figured by Cope, op. cit. [1891.2] 

 this measurement is about 18 millimeters, and in the symphysis of the jaw (No. I, 

 also figured) a like measurement given, by the same authority, as 27 millimeters , 

 should be 22 millimetei s. 



>' First premolar in Cope's description of M. angustigenis. 



