EVOLUTION OP THE SKULL AND TEETH OF EOCENE TITANOTHERES 



327 



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postorbital processes. Between the orbits the skull is 

 broadly convex. The nasals extend 175 millimeters 

 anteroposteriorly; posterolaterally they send down a 

 broad flange beneath the adjoining parts of the max- 

 illaries, lacrimals, and frontals. This flange is not 

 (as in Manteoceras, etc.) largely covered by the forward 

 extension of the frontals but appears in side view as a 

 V-shaped area lying between the frontals and the 

 maxillaries, a feature which is very characteristic 

 of Palaeosyops and Limnohyops. The maxillaries 

 rise in front of this point «<^'5*?>. 



and arch over the maxillary 

 notch. Another character- 

 istic feature is that the 

 nasals diminish toward the 

 tips both in width and in 

 the depth of the lateral 

 decurvature. Still another 

 feature is the broad en- 

 trance of the maxillaries 

 below the anterior portion 

 of the zygomatic arch. 



Horn swellings: By far 

 the most important feature 

 of this skull is the lateral 

 horn swelling (fig. 275) on 

 each of the frontals behind 

 the nasofrontal sutures, 

 which are entirely smooth. 

 They would certainly never 

 have been observed if atten- 

 tion had not been directed 

 to this particular region of 

 the skull by the distinct 

 and rugose horn rudiments 

 seen in a subsequent stage 

 (P. rohustus). 



The type stage (muta- 

 tion): A more progressive 

 stage is the type cranium. 

 Am. Mus. 1544. In this 

 male skull the frontonasal 

 horn swellings are so slight 

 that they can barely be 

 distinguished. This is a 



very important point because in the female skull just 

 described the horn swellings are quite apparent. This 

 fact, in connection with corroborative evidence in 

 other phyla, tends to prove that in their inception the 

 horns are not sexual characters. The supratemporal 

 crests at the narrowest point are separated 36 milli- 

 meters by the broadly plane vertex of the skull, which 

 passes uninterruptedly into the occiput by a gentle 

 curve, there being no definite supraoccipital border. 

 Immediately behind the orbits the vertex measures 

 136 millimeters transversely; between the orbits, 119 

 101959— 29— VOL 1 24 



transversely. The frontoparietal, intei'frontal, and 

 frontonasal sutures are all closed by age. The nasals 

 narrow from 76 millimeters at the broadest point 

 posteriorly to 47 at the tips. In the palatal aspect 

 we observe that the palate is relatively short and broad 

 and not decidedly arched, the horizontal plates of the 

 palatines being abbreviated. The postnarial space is 

 relatively short and deep but less excavated than in 

 Telmatherium ultimum. A relatively narrow bridge 

 of bone (14 mm.) separates the foramen ovale from 



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yf 





Figure 276. — Type skull of Palaeosyops leidyi 

 Am. Mus. 1544; upper part of Bridger formation, level C or D. Drawings by R. Weber. One-sixth natural size. 



the foramen lacerum medium. The postglenoid proc- 

 esses are transversely extended but less decidedly so 

 than in T. ultimum. A very characteristic feature is 

 the broad triangular plate formed by the basioccipital 

 and basisphenoid, slightly keeled in the median line, 

 with a prominent rugosity (insertion of rectus capitus 

 and constrictor muscles) at the junction of the basi- 

 occipital and basisphenoid and a very narrow bridge 

 between the condylar foramen and foramen lacerum 

 posterius. This is very different from the more 

 elongate and laterally compressed region in Tel- 



