EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



529 



Measurements of Menodus trigonoceras and Allops serotinus, in 

 millimeters 



Pi-m3 



Pi-p< 



Mi-m^ 



Pmx to condyles 

 Zygomatic index 



Nasal length 



P*, ap. by tr 



M^ ap. by tr 



" Am. Mus. 6356. 



Materials. — This important intermediate stage is 

 represented by the type and paratype skulls in the 

 American Museum (Nos. 6355 and 6356, Cope collec- 

 tion) ; also by two skulls in the National Museum 

 (Nos. 4257, 1219). Nat. Mus. 4257 is definitely 

 recorded by Hatcher as from the upper level of B, the 

 middle Titanotherium 

 zone, a fact of extreme 

 importance, as helping 

 to determine the geo- 

 logic level of this spe- 

 cies. Another skull 

 (Nat. Mus. 4701, ? ) 

 agrees in measurement 

 with M. proutii but is 

 recorded from the base 

 of C; it thus may be 

 a female of M. trigono- 

 ceras. There is no jaw 

 positively associated 

 with M. trigonoceras; 

 a referred jaw is Am. 



Mus. 1007, which is transitional to M. giganteus in 

 its measurements. 



If measurements are reliable as indications of specific 

 affinity we should have to add to this species the skulls 

 Am. Mus. 1066, cr', 1067, ? , and the jaw Am. Mus. 1007, 

 which were formerly included under M. giganteus.^ 



Characters oj the type and paratype of M. trigono- 

 ceras. — Cope's type (Am. Mus. 6355) fortunately is a 

 skull in which all the superior portions are fairly pre- 

 served, as shown in Plates CXXVIII and CXXIX; 

 the principal missing parts are the premaxdlaries, maxil- 

 laries, and occipital condyles. A single molar tooth, 

 m^, shows that this type skull is young or in an early 

 stage of growth, while the paratype skull (Am. Mus. 

 6356) is entering the seventh stage of growth. 



This disparity of age explains some differences be- 

 tween these two specimens, especially in the zygomatic 

 arches, which are wholly attributable to growth or age. 



The type skull (No. 6355) belongs to a young indi- 

 vidual; the nasals are long and rather thin, with 



nearly parallel sides, truncate distally. The horns are 

 short, widely divergent, with trihedral section and a 

 well-defined intermediate connecting crest, which is 

 placed at the back part of the horn. The cranial 

 vertex is of moderate width and extends backward 

 into a slightly expanded occiput with lateral pillars, 

 extending into rugose summits; the zygomatic arches 

 are slender in vertical section, but they show marked 

 expansion. The third superior molar bears a small, 

 very sharply defined hypocone, a distinctive phyletic 

 character of this series. 



The paratype skull consists of the anterior portion 

 (Am. Mus. 6356) of a fully adult individual in the 

 seventh stage of growth. The more advanced age 

 presents us with the adult characters of this species, 

 especially the thickening and rugose expansion of the 

 nasals distally, the thickening and moderate expansion 

 of the zygomatic arches, the well-developed preorbital 

 and postorbital processes. 



Dentition. — The specific identification of the para- 

 type with the type is rendered more positive by the 



Figure 439. — Skull of Menodus trigonoceras 



Side view. Tiiis skull (now in the Munich Museum, formerly Am. Mus. 1066) is vertically crushed, so that the nasals are tilted 



upward, the horns crushed downward, and the vertical diameters of the orbit and parietal region lessened. One-sixth natural size. 



close similarity in the structure" of the third superior 

 molar in both skulls. This tooth has a sharp and dis- 

 tinct hypocone. Other dental characters have weighty 

 specific value (Pis. CXXXI, CXXXII). The techni- 

 cal formula, I^, C^, P^, M^, fails to convey an idea of 

 the vestigial character of the incisor teeth, the crowns 

 of which barely break through the bone of the incisive 

 border and certainly did not cut through the gum; 

 thus the premaxilla of this animal should be described 

 as functionally edentulous. The sex of the paratype is 

 apparently female, the canine being rather slender and 

 elongate. The maxillary teeth are rather progressive 

 in development; the first premolar exhibits a rudi- 

 mentary tetartocone, or four cusps in all. The tetarto- 

 cones on p^, p'' are well developed but still much 

 inferior in size to the deuterocones; the premolars are 

 further distinguished by pronounced internal and 

 external cingula. Throughout the premolar-molar 

 series we observe a deep pit in the midvalley, just 

 internal to the ectoloph, which is homologous with the 



