EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



303 



9. The sagittal crest of Palaeosyops is lower, broader, 

 and passes more rapidly into the temporal ridges, 

 whereas in Limnohyops as in Telmatherium the crest 

 is higher and thinner and extends well forward before 

 spreading into the temporal ridges. 



10. The male jaws of Palaeosyops are at once recog- 

 nized by (a) the more prominent, massive chin and, 

 as seen from below, the short, depressed area for 

 the digastric muscle on the posterior symphyseal 

 line, features that contrast with the longer, more slop- 

 ing chin of Limnohyops and its elongate median fossa 

 for the digastric; (6) the massive breadth of the 

 Palaeosyops rami, as seen from below, in contrast with 

 the somewhat more slender inferior borders of the jaw 

 in Limnohyops; (c) the extremely distinctive base of 

 the anterior border of the coronoid process which in 

 Palaeosyops is very broad and in progressive stages 

 deeply hollowed out in front, whereas in progressive 

 stages of Limnohyops it is somewhat narrower, less 

 deeply excavated, and lies more to the outer side of 

 the line of the molar teeth. 



Additional means of distinction are set forth in the 

 descriptions of the genera and species. 



W. K. Gregory has observed that the above and 

 other differences are in part quantitative; they are 

 differences in the proportion of one and the same 

 character, as in the form of the nasals, of the sub- 

 orbital bar, and of the sagittal crest. The divergence 

 is far less than that seen in the modern genus Cervus, 

 for example. It may be noted also that certain of 

 the numerical differences are variable; for example, 

 the hypocone on m^. The known forms of Limnohyops 

 are rather slender; thus a male jaw of this animal 

 would resemble a female jaw of Palaeosyops. 



Limnohyops Marsh 



Plates LVI, LVII, LX, LXII; text figures 29, 87, 96, 115-117, 

 219, 254, 256, 258-266, 274, 484, 485, 510-514, 516, 518-523, 

 525, 527-532, 538, 672, 685, 686, 690, 701, 714, 717, 722, 

 723, 745, 760 



[For original description and type reference see p. 170; for skeletal characters see p. 605] 



Generic characters. — Brachy cephalic; grinding teeth 

 persistently brachyodont; conules on the molars per- 

 sistent, usually lophoid; third superior molar sub- 

 quadrate and usually with distinct hypocone. Pro- 

 portions of skull and skeleton moderately robust. 

 Manus slender. Five sacral vertebrae (type). 



Geographic and geologic distribution. — Limnohyops 

 has thus far been found only in the geologic levels 

 B, C, and D of the western or Bridger Basin (see 

 geographic map on p. 8). As compared with Palaeo- 

 syops the materials representing this genus or sub- 

 genus are limited; we can not therefore trace at present 

 the successive stages of its evolution. It is subdi- 

 vided into five species — L. Inevidens, L. priscus, Z. 

 laticeps, i. matthewi, L. monoconus. 



Resemblances to Palaeosyops. — From our present 

 knowledge the geologically early species L. laevidens 



and L. priscus are so close to the type of Palaeosyops 

 (P. paludosus) that they might readily be embraced 

 within one and the same genus. In fact, material at 

 first referred by the present author to L. priscus now 

 appears to belong to P. paludosus, which is itself so 

 primitive that it may almost be regarded as the an- 

 cestor of Limnohyops. However, the sum total of 

 the distinctions between these animals — in external 

 form, color, and habits — was probably very consid- 

 erable, and as we progress into geologically higher 

 stages the cranial and dental differences become more 

 apparent, as summarized below. 



Materials. — Besides the admirable type specimen 

 from Bridger C 4 in the Yale Museum, on which 

 Marsh founded the genus and species, American 

 Museum parties have found five specimens of L. 

 laticeps in levels Bridger C 4 and D 2. In the lower 

 level of Bridger B 2 occurs the type of L. laevidens 

 Cope, and here we have also found two specimens of 

 the somewhat more progressive stage L. priscus. 

 At present the species L. matthewi and L. monoconus 

 are represented only by a single specimen each, and 

 it is noteworthy that these also are of lower Bridger 

 age. Thus our knowledge of Limnohyops at present 

 rests on portions of about 16 individuals. 



Chief progressive distinctions from Palaeosyops. — 

 (Compare pp. 302, 618, vertebrae; p. 612, limbs and feet.) 

 So far as we know at present Limnohyops is distin- 

 guished by somewhat more slender proportions. The 

 skull in the larger species is equally broad but less 

 massive ; the j aws are decidedly less massive. The long 

 bones of the limbs referred to L. laticeps are practically 

 of the same length as those of the contemporary 

 Palaeosyops leidyi, but the foot bones of the manus 

 of Limnohyops appear to be shorter (figs. 512, 520). 

 Limnohyops may be described briefly as a relatively 

 light-limbed, broad-skulled, short-footed type. 



As we have already given many of the details by 

 which Limnohyops in its advanced stages may be 

 distinguished from Palaeosyops, it is only necessary 

 to summarize its chief diagnostic characters. 



Cranium: (1) The skull of Limnohyops has a concave 

 instead of a convex forehead (fig. 256); (2) it has a 

 high, thin sagittal crest; (3) the nasals are relatively 

 broad anteriorly; (4) the suborbital bridge of the 

 malars is shallow, narrow, and more or less quad- 

 rangular in section or broadly depressed, with a 

 rounder outer border, and the fiange for masseteric 

 insertion is not extended so far forward as in Palaeo- 

 syops; (5) the splint of the maxilla extends backward 

 under the side of the malar; (6) only slight promi- 

 nences indicate the osseous horn areas, and no horn 

 rugosities have been observed; (7) the jaws have more 

 slender rami, the chin is sloping, the digastric fossa 

 is elongate, deep, and sharply defined, the lower border 

 is less thickened below the grinders, the coronoid base 

 is less broad and flaring anteriorly, when seen from 



