390 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Cranial elongation with age: There is considerable 

 evidence that cranial elongation is not only a progres- 

 sive but an age character — that is, one which appears 

 in advanced years and through the "law of accelera- 

 tion" will appear in earlier years of subsequent gener- 

 ations. For example, the space between the glenoid 

 fossa and m^ elongates with age, and correlated with 

 it is the elongation of the ramus of the jaw between ma, 

 the anterior border of the coronoid process, and the 

 condyle. 



pas P"~ 



Figure 327. — Tj'pe skull of Mesatirhinus petersoni 



One-fourth natural size. Am. Mus. 12184, Cattail Springs, Bridger Basin, Wyo., 

 Bridger D 3. Ai, Side view; Ai, top view. 



Slcull. — The general characters of the skull are as 

 follows: (1) Skull as a whole elongate, with consider- 

 able space between the glenoid process and the occipital 

 condyles, leaving the auditory meatus open, as con- 

 trasted with the contemporary Palaeosyops, in which 

 this space is abbreviated; (2) zygomatic arches rela- 

 tively straight, slender, and gently arched outward; 

 (3) on the malars an infraorbital shelf, which ap- 

 parently increases progressively; (4) sagittal crest 

 sessile and narrow as compared with LimnoJiyops 

 laticeps; (5) nasals long, expanding and decurved dis- 

 tally; (6) premaxillary symphysis more abbreviate 



than in Telmatherium; (7) face moderately bent upon 

 cranium, parietals convex in side view; (8) postorbital 

 process of malar above posterior part of m^. On 

 comparing the side views of Telmatherium cultridens 

 and of Mesatirhinus petersoni we see that in the 

 former the premaxillary is stouter, vertically deeper 

 anteriorly, and extends posteriorly farther up on the 

 maxillary. In T. ultimum this is much more pro- 

 nounced. In Mesatirhinus, in correlation with the 

 smaller incisors and more slender maxUla, the pre- 

 maxillary is shallower vertically, and the sym- 

 physeal surface is more delicate. 



The skull of members of Mesatirhinus petersoni ex- 

 hibits many marks of general affinity to those of their 

 collateral relative Manteoceras manteoceras. Among 

 these are (1) the shape of the symphyseal union of the 

 premaxillaries ; (2) the narrowing of the postnarial 

 space between the pterygoids; (3) the sutural rela- 

 tions of the nasals, frontals, maxillaries, malars, and 

 lacrimals, as seen in side view, with the exception of 

 the position of the horn rudiment; (4) the concavities 

 at the side of the face slightly above and in front of 

 the orbits; (5) just above these concavities the promi- 

 nent convexities of the nasals at their junction with 

 the frontals above the orbits, extremely interesting as 

 a very early stage of horn evolution and prophetic of 

 the distinct horn base of Dolichorhinus; (6) presence 

 of a long and narrow pit in the anterior portion of the 

 sagittal crest. 



Among the most significant resemblances to Man- 

 teoceras are also the similarity in the base of the 

 craniima; the slender zygomata, constricted back of 

 the orbit; and the underlying similarity in the denti- 

 tion in spite of differences of proportion. 



The skull differs markedly from that of Manteo- 

 ceras, however, in the presence of infraorbital shelves 

 and in its greater dolichocephaly. It also differs from 

 Manteoceras in the shape of the occiput, shape of the 

 skull top, and especially in the dentition. Its closer 

 affinities, therefore, are with Dolichorhinus. 



The detailed characters of the teeth exhibit a direct 

 dolichocephalic adaptation of those of the Manteo- 

 ceras type. The community of type, again, is due to 

 a community of ancestry, the two lines running 

 together perhaps prior to Wind River and Huerfano 

 time. 



More in detail: The superior view of the skull 

 (fig. 328) exhibits the characteristic anterior expansion 

 of the nasals, which measure transversely (Am. Mus. 

 1556, M. petersoni) anterior region 59 millimeters, mid- 

 region 43, posterior region 84; the total length is 167. 

 The nasals are separate anteriorly but firmly coalesced 

 posteriorly; the lateral convexity just in front of their 

 junction with the frontals (figs. 327, 328) represents the 

 rudimentary stage in the evolution of the horn. The 

 frontals are expanded above the orbits (91 mm., tr.), 

 gradually contract posteriorly, and are bounded by the 



