50 



are broken off (that of the right tooth being restored in fig. 5 of plate VI), and the right first 

 premolar is lost from its alveolus. 



Compared with Megacerops angustigenis (Cope), (this volume part I, p. 13, plate V, 

 fig. 2 and plate VI, figs. 2, 2a, M. primilivus differs in the following respects : — There are 

 six incisors instead of four, and the breadth of the jaw between the canines, which are of 

 smaller diameter, is relatively greater ; the diastema between the canine and the first premo- 

 lar is twice as long ; the symphysis is of greater length, reaching back to a point almost in line 

 with the posterior edge of the fourth premolar (in angustigenis in line with the anterior root 

 of the corresponding tooth) ; the exterior cingala are much less developed ; the coronoid 

 process is shorter. In both species the premolars have reached about the same stage of 

 development toward the molar pattern, and the molars have very much the same propor- 

 tions. In angustigenis the internal cingula are partially developed. In primitivus the man- 

 dible is shorter, proportionately deeper, and not so thick in the neighbourhood of the alve- 

 olar border posteriorly. 



Keeping in mind the differences due to sex in Titanotheres generally, and the apparent 

 variability, both specific and individual, of certain dental characters, such as the degree of 

 development 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 full 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. That the 

 species is already represented by cranial fragments from the Cypress hills, found separately, 

 is quite possible, and further material will probably prove the identity of some of the species, 

 from this locality, described under different names. 



Measurements of mandible of M. primitivus. 



MM. 



Length of ramus 475 



Depth of same at posterior end of fourth premolar 74 



" " " second molar 81 



" " from tip of coronoid process to lower border 247 



Maximum thickness of same beneath third molar 46 



Length of symphysis 1 44 



Distance apart of inside surface of base of canines* 31 



Length of premolar series 103 



" molar series 183 



tv c . , , | antero-posterior 18 



Diameter or canines at base . . - r , a 



I transverse lb 



lv . c , , ( antero-posterior 26 



Diameter ot second premolar . . r , „ 



I transverse lo 



Diameter of third premolar. . . •>' Uo 



r I transverse ~A 



Diameter of fourth** premolar ' „,. 



I transverse 1 1 



* In the mandible of -1/. anguttigeni* (Xo. II) figured by Cope, op. cit., this measurement is about 18 mm., and in the 

 symphysis of the jaw (Xo. I, also figured) a like measurement given, by the same authority, as 27 mm., should read 22 mm. 

 ** First premolar in Cope's description of .1/. angiusligenis. 



