366 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



the junction with the frontals. The midlength of the 

 nasals is 174 millimeters, as compared with 164 in the 

 much smaller M. megarJiinus skull, showing that while 

 the cranial portion of the skull has greatly increased 

 in length, the nasal bones have not increased in length 

 so fast as they have in MesatirMnus ; in other words, 

 a retardation in the progressive lengthening of the 

 nasals is observable and is more strongly expressed 

 in M. uintensis; and this points toward the transfor- 

 mation of the Manteoceras skull into the Oligocene 

 titanothere type. In the contemporary D. Jiyogna- 

 thus, on the other hand, which does not lead into an 



Figure 304. — Type skull of Manteoceras 

 manteoceras 



Top view. About one-fifth natural size. Am. Mus 1569. 

 Washakie Basin, Wyo., level A ( UMatherium-Manteoceras- 

 Mesatirhinvs zone). After Osborn, Am. Mus. Nat. Hist^ 

 Bull., vol. 7, flg. 8, 1895. H, horn swelling. 



Oligocene titanothere, the nasals attain the extraor- 

 dinary length of 290 millimeters. 



2. The frontal horn caps in the Eocene as in the 

 Oligocene titanotheres overlap the outer sides of the 

 nasals, so that the horn bases present upwardly and 

 outwardly. 



3. The orbits (Am. Mus. 1570) are 133 millimeters 

 apart, and from the prominent triangular postorbital 

 processes the narrow but distinctly rugose supratem- 

 poral ridges converge backward into a broad, laterally 

 expanded frontoparietal plate which prophetically 



represents the flattened summit of the Oligocene 

 titanothere cranium. This plate flares laterally 

 over the temporal fossae, as in many other species 

 of titanotheres. Behind this point the supratemporal 

 ridges converge to form a deep midparietal pit, which 

 is apparently homologous with the vestigial pit ob- 

 served in several species of the Oligocene Brontops; 

 the supratemporal ridges again diverge, leaviag a 

 narrow groove between the paired sagittal crest, 

 which is from 18 to 29 millimeters in width. 



4. A very important feature of the superior view, 

 seen also in MesatirMnus, is the comparatively oval 

 form of the openings left by the zygomatic arches, 

 and the great backward stretch of the floor of the 

 temporal fossa from the j miction of the zygomata 

 with the skull of the occiput. 



The palatal view of the skull (fig. 303, B), best seen 

 in three specimens La the American Museum, Nos. 

 1545, 2353, 1570, exhibits the following principal 

 characters: (1) The base of the cranium, the mid- 

 cranial region (postglenoid to orbit), and the face are 

 all relatively longer than in Palaeosyops but shorter 

 than in MesatirMnus; (2) the posterior nares open 

 about the middle of the skull, opposite the interval 

 between m2, ms; (3) the hard palate is gently arched; 

 (4) the posterior narial space is elongate and nar- 

 rowed by the decided median convexities of the pala- 

 tines; (5) the pterygoids are greatly reduced as thin, 

 elongate plates; (6) the relations of basicranial bones 

 and foramina are as shown in Figure 303, B; (7) there 

 are paired rugosities on the basisphenoid for attach- 

 ment of the recti capitis muscles; (8) there is an 

 elongate bridge (35 mm.) between the foramen ovale 

 and foramen lacerum medium; (9) the inner portions 

 of the occipital condyles are borne on the basioccipitals; 

 (10) the paroccipital process is delicate. 



The anterior view of the cranium (fig. 305, B) 

 clearly illustrates the decurved and thickened margins 

 of the nasals, the prominence of the frontonasal 

 horn, the deep lateral facial concavities, the character- 

 istic structure of the premaxillaries, and the deep 

 premaxillary symphysis. 



In the lateral view (figs. 307, 308) the superior 

 profile is incipiently saddle-shaped, as in the Oligocene 

 titanotheres, and we note that the skull descends from 

 the occiput to the midparietal region, then arches 

 gently upward to a point directly above the orbits, 

 and then descends to the tip of the nasals. This 

 facial convexity, combined with the lateral preorbital 

 concavities, contributes to and is correlated with the 

 prominence of the frontonasal horn. The facial 

 concavity profile is similar to that of MesatirMnus 

 and is totally different from the transversely convex 

 preorbital section of Palaeosyops. Possible but doubt- 

 ful evidence of a progressive shortening of the face 

 is observed in the variable position of the infraorbital 

 foramen. In most of the skulls (Am. Mus. 1570, 1511, 

 1587, 1545) there is a broad bridge of bone over 



