EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



311 



Limnohyops laticeps Marsh 



Plates LVII, LXII; text figures 87, 92, 261, 264-266, 511, 531, 

 532, 714, 760 



[For original description and type references see p. 160. For skeletal character 

 see p. 6181 



Type locality and geologic horizon. — Bridger Basin, 

 Wyo. Marsh's Fork, the level of which is not cer- 

 tainly known, is the type locality. The American 

 Museum specimens closely resembling the type are 

 from Bridger C 4, D 1, and D 2. 



Specific characters. — Of intermediate size; p'-m^, 

 153 millimeters; p^-m', 139; second and third in- 

 ferior premolars of more advanced type than in L. 



FiGiTRB 264. — Skull of Limnohyops laticeps 



Top view. One-fotirth natural size. Yale Mus. 11000 (type). Bridger 

 Basin, Wye; upper (?) part of Bridger formation. 



priscus; p^ with well-developed tritocone; m' with a 

 large hypocone (fig. 265). Condyles to incisive border 

 410 millimeters (estimated); breadth across zygomata 

 320; smooth and extremely rudimentary horn swell- 

 ings on nasofrontal sutures. Cephalic index 75 (esti- 

 mated). 



This was one of the earliest of the Palaeosyopinae 

 to be described, and for a long time it was not clearly 

 separated from the genus Palaeosyops. The most dis- 

 tinctive character assigned by Marsh was the hypocone 

 oa the last superior molar (fig. 265). Many of the 

 distinctive cranial characters were clearly pointed out 

 101959— 29— VOL 1 23 



by Earle. Additional materials in the American Mu- 

 seum collections enable us to fully define this species, 

 especially from the full characters of the skull, the 

 carpus, and the manus. 



Materials. — L. laticeps is represented by two speci- 

 mens — by the type cranium and parts of the skeleton 

 (Yale Mus. 11000), belonging to an individual not 

 fully grown, and by Am. Mus. 11710, a fragment of 

 the maxilla containing two molars, from level D 2, 

 Bridger. 



Specific characters of the type. — In addition to the 

 specific characters enumerated above the occiput is 

 moderately high (144 mm. above the condyles, 118 

 above the foramen magnum) ; the condyles are moder- 

 ately broad (95 mm.); the mesostyle on p*, which is 

 seen as a shadow rudiment or rectigradation in L. 

 priscus, is here quite distinct; in p^ the tritocone is 

 much larger and more distinct than in L. priscus; m' 

 as in L. priscus is small (ap. 24 mm., tr. 27); m^ is 

 much larger than in that species; the metaconules 

 are larger than in L. priscus and confluent internally 

 with the hyipocones; there 

 is little or no diastema be- 

 hind the canine. 



The type slcull. — Our 

 knowledge of the skull is 

 based mainly on the crushed 

 but very complete type 

 cranium of L. laticeps (Yale 

 Mus. 11000), which gives 

 us the principal characters. 

 (See fig. 264.) (1) The pro- 

 portions of the skull are 

 approximately the same as 

 in P. leidyi — namely, 410 

 millimeters in length and 

 310 across the zygomata. 

 (2) The skull of L. laticeps 

 is distinguished from that of L. priscus and approaches 

 that of P. leidyi in the slight narrowing of the nasals 

 anteriorly: posteriorly they measure 53 millimeters in 

 width ; anteriorly they diniinish to 43 millimeters, being 

 still much broader anteriorly than in P. leidyi. Other 

 distinctions from P. major and P. leidyi are found in the 

 following principal characters : (3) The sagittal crest is 

 very high, extending 65 millimeters above the brain 

 case, and thin at the summit (9 mm.), extending for- 

 ward a considerable distance (103 mm.) before the 

 crest begins to spread into the supratemporal ridges, 

 whereas in the least progressive specimen of P. leidyi 

 described below the thinnest portion of the crest 

 measures 13 millimeters and begins to expand rapidly 

 into the plane of the vertex; (4) the occiput as seen 

 from behind is well defined by a sharp crest and is 

 rounded superiorly, extending 118 millimeters above 

 the foramen magnum and 125 millimeters trans- 

 versely; (5) the postglenoid and post- tympanic proc- 

 esses are slightly separated; (6) the zygomata arch 

 widely, the malars being compressed inferiorly and 



Figure 265. — Third right up- 

 per molar of Limnohyops 

 laticeps 



Natural size. Am. Mus. 11710, re- 

 versed. Henrys Fork, Lone Tree, 

 Bridger Basin, Wyo.; Bridger for- 

 mation, level D 2. 



