EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



575 



between the males and females is now still more 

 marked, the latter being apparently represented by 

 specimens referred to the species B. {Menodus) 

 peltoceras Cope. 



Materials. — Besides the noble type skull of Titanops 

 curtus in the Yale Museum (No. 12013) this species 

 is represented by a pair of horns in the Harvard 

 Museum and by a large, fully adult skull (Nat. 

 Mus. 1211) and the skull of an old bull (Nat. Mus. 

 4946) in the National Museum. Both these skulls in 

 the National Museum are recorded from the top 

 level of the upper TitanotJierium zone (Chadron C). 

 In the American Museum there is fortunately a 

 small female skull, No. 1005. Cope's type of 

 Menodus peltoceras (Am. Mus. 10719) consists simply 

 of the great connecting crest supporting a pair of 

 low horns. 



Detailed description of the type sJcull. — The type 

 male skull (Yale Mus. 12013, Pis. CLXXVIII- 

 CLXXX) is in the seventh stage of growth. The 

 horns have therefore not attained their maximum 

 length, and the connecting crest is comparatively low 

 and rounded. The horns are placed well forward 

 and vertically overhang the canines. The external 

 ridge has increased, while the anterior ridge has 

 practically disappeared (PL CLXXIX; figs. 457, G; 

 475-478). The section of the horns is now almost 

 planoconvex, the inferior face being strongly convex 

 and the posterior face almost plane; at the summits 

 the horns exhibit a rugose lateral expansion suggestive 

 of that which is so strongly marked in B. ramosum. 

 Another progressive feature is the very sudden expan- 

 sion of the zygomatic arches into a broad and relatively 

 shallow convexity (Pis. CLXXVIII, CLXXX). The 

 vertex of the cranium is bounded by a lateral crest 

 with a rugose border overhanging the temporal fossa. 



Dentition. — The superior canines are short, re- 

 curved, with a broad posterior cingulum. The pre- 

 molars exhibit well-separated internal cones, es- 

 pecially on p' and p^, there being a distinct cleft 

 between the deuterocones and the tetartocones. The 

 first superior premolar is indicated by an alveolus on 

 the left side. In the molar-premolar series the 

 enamel is crenulate on the sides of the internal cones. 

 On m' the hypocone is fairly prominent but not 

 separate from the cingulum. 



Age and growth characters. — As above noted the type 

 male skull, being in the seventh stage of growth, does 

 not exhibit either the maximum length of the horns 

 or the maximum development of the connecting crest. 

 In the fully adult male skulls in the National Museum 

 (Nos. 4946 and 1211), which are in the eighth stage of 

 growth, we have finely illustrated the skull characters 

 of the old bulls. The connecting crest of No. 1211, 

 which is the oldest and most progressive skull, is now 

 extremely deep and descends by an almost straight 

 line to the tips of the nasals, which are now only 40 



millimeters in length. The horns are very long 

 (355 mm.), recurved, with the characteristic strongly 

 convex section in front and slightly convex section 

 behind. The second old bull in the National Museum 

 (No. 4946) is somewhat less progressive, the horn sec- 

 tion (fig. 475, B) being very convex anteriorly and less 

 convex posteriorly. The horns are relatively broader 

 and more flattened posteriorly, and the nasals are still 

 more abbreviate (65 mm.). The horn of this speci- 

 men has been broken off and partly regenerated during 

 life, a fact which appears to show the value of the 

 horns in contests between the males. 



A specimen in the Harvard Museum (No. 1004) 

 represents a much smaller animal in an earlier stage of 

 evolution, in which the horns are still convex on the 

 posterior surfaces. All these specimens agree with 

 B. gigas in the backward extension of the occiput 

 behind the zygomata, in the inward flange of the 

 zygomata, and in numerous other details of character. 



Other features in the National Museum skull (No. 

 4946) are the following. It appears that skull growth 

 continued even after the teeth were much worn down. 

 Added to the progressive feature of the lengthening 

 and flattening of the horns and the abbreviation of the 

 nasals, marked in this bull, is the incipient develop- 

 ment of a narrow ridge on the outer side of the horns, 

 an exaggeration of the "external or malar ridge"; it 

 does not extend very far but is marked in old indi- 

 viduals. This "malar ridge" is prophetic of the con- 

 tinuous external ridge down the entire side of the horn 

 in the higher phyletic stage, B. platyceras. The 

 structure of the zygomatic arch is especially interesting; 

 immediately behind the orbit it consists of a vertically 

 compressed plate. A variation of interest is the 

 small tuberosity on the inner side of the horn, sug- 

 gestive of the internal hornlet of Diploclonus. 



Carnegie Museum specimen. — A fine pair of horns 

 in the Carnegie Museum (No. 560) which are referred 

 to this species have an external height of 365 milli- 

 meters and a basal anteroposterior diameter of 103 

 millimeters. They are of special interest as showing 

 a pair of secondary horns or hornlets, which consist of 

 elongate oval swellings 78 millimeters long at the base 

 and about 22 millimeters high, located on the internal 

 border of the main horns. 



Male slcull referred hy Cope to " Sy7nhorodon" iucco 

 (Am. Mus. 6346). — As we have seen above, the actual 

 type of " Symhorodon" hucco is a skull (Am. Mus. 6345) 

 which belongs to Megacerops hucco. Another speci- 

 men described and figured by Cope as "Symhorodon" 

 hucco (Am. Mus. 6346) is probably referable to B. 

 curtum. The buccal processes are very broad (656 

 mm., estimated), arching suddenly outward, deep and 

 biconvex in section. The occipital crest is backwardly 

 extended. There is a median vertical ridge over the 

 foramen magnum, and a pair of rugosities on either 

 side of the median line. The basisphenoid is smooth. 



