544 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Megacerops coloradensis Leidy 



Text figures 164, 448 

 [For original description and type references see p. 20S| 



Tyfe locality and geologic Tiorizon. — Colorado; Ti- 

 tanotlierium zone. Exact locality and level unknown. 

 Specific characters. — Nasals long (110 mm., esti- 

 mated), sharply decurved and tapering distally; horns 

 cylindrical with slightly flattened tips, external length 

 175 millimeters (estimated). Connecting crest very 

 low. 



Materials. — The type specimen, consisting of the 

 coossified nasals and horns, has been lost, and the 

 characters of the genus rest upon Leidy's original de- 

 scription and figures and on the model (Am. Mus. 

 9018, fig. 448) made from them. 



Transitional referred specimen, Brontotherium ticho- 

 ceras. — In the National Museum there is a beautifully 

 preserved skull (No. 8313, 

 with associated lower j aw) 

 which in horn structure 

 recalls the type of M. 

 coloradensis, except that 

 the nasals and horns are 

 somewhat shorter. This 

 skuU is of extraordinary 

 interest inasmuch as it 

 combines the nasals, horns, 

 and absence of connec- 

 ting crest which are char- 

 acteristic of Megacerops 

 with the large incisors 

 and canines which had 



T? AAo a„+;„„„.,„^ hitherto been regarded 



Figure 448. — Sections and _ *= 



contours of nasals and horns as characteristic only of 



of Megacerops coloradensis. BrontotJierium. The pre- 



Ara. Mus. 9018, a model made trom molars are relatively Small, 



Leidy's figures and from measurements . " . 



of iiis type, wiiich lias been lost. One- and the internal cmgula 



seventh natural size. j^^^^ ^^^^^^^ completely 



disappeared, much more than in BrontotJierium and even 

 more than in most Megacerops skulls. The measure- 

 ments of this skuU approach those of the type of 

 Megacerops hucco except that the basilar length is 

 greater, the zygomatic width and horn length are 

 less. It should also be compared with Brontotherium 

 hatcheri, but it differs from that type in its shorter 

 horns, absence of connecting crest, and absence of in- 

 ternal cingulum on the premolars. 



Megacerops bucco (Cope) 



{Symhorodon bucco Cope, 1873; "Symhorodon iorviis" Osborn, 

 1902) 



Plates CXLIV, CXLV, CLIII-CLVI; text figures 169, 170, .378, 



392, 393, 399, 449, 719, 744 



[For original description and type references see p. 2121 



Type locality and geologic horizon. — Cedar Creek, 

 Logan County, Colo.; Titanotherium zone. 



Specific and generic characters. — I^l^j P^. Superior 

 incisors probably absent; premolars with cingula 

 vestigial or wanting, tetartocones nearly as large as 

 deuterocones and more distinct then in M. copei, 

 a faint mesostyle upon p*. Skull shows progressive 

 increase in size over M. copei, length nasals to occiput 

 750 millimeters, premaxillaries to condyles 665 (esti- 

 mated); extremely brachycephalic, index 100 (esti- 

 mated); nasals thin, medium in length, broad (90 by 

 130 mm.); no connecting crest between horns; horns 

 c? short (185 mm., estimated); zygomata broadly ex- 

 panded; occipital pillars expanding, with wide superior 

 rugosities. 



SlcuU and jaw selected hy Cope as types of Symhorodon 

 hucco. — The type skull of S. hucco Cope includes a 

 fairly preserved skull (Am. Mus. 6345a) with enor- 

 mous zygomatic expansions. The jaw placed with 

 this skull by Cope appears to belong to another 

 phylum. The premaxillaries are wanting. All the 

 maxillary teeth on both sides are preserved with the 

 exception of p^ The horns and the anterior portions 

 of the orbits have required much restoration but 

 serve to afford some very distinctive characters. The 

 skull is readily distinguished as a Megacerops by the 

 entire absence of the connecting crests between the 

 horns, which are set widely apart but so damaged 

 that a perfect basal section can not be made. The 

 nasals are very thin and of medium length. The 

 zygomata arch very widely and exhibit a flattened 

 section which is even more extreme than that of 

 Brontotherium gigas and quite distinct from the con- 

 vex section of the skull Am. Mus. 6346, also referred 

 by Cope to S. hucco but regarded by us as pertaining 

 to a species of Brontotherium, probably B. curium. 

 Posteriorly the cranium broadens out into the occiput, 

 but it is important to note that this breadth and the 

 peculiar zj'gomatic section are altered by crushing. 



Dentition. — The animal is in the eighth stage of 

 growth, all the internal cones of the teeth being worn 

 except upon m^. The external cingulum is obsolete 

 throughout the grinding series, as in M. acer. The 

 internal cingulum is entirely lacking on p^ and is 

 very slightly marked on p^. It is also wanting on the 

 inner sides of the molars. As a marked progressive 

 character, the internal cusps of the premolars are 

 robust and well roimded. The hypocones on m^ m- 

 are prominent and bidge on the lingual side. The 

 hypocone is well developed on m^, triradiate in form, 

 and connected with the cingulum by only a slight 

 posterior ridge. 



Female slcull of M. hucco in the National Museum. — 

 The skull Nat. Mus. 4705 is that of an aged female in 

 the ninth stage of growth, the horns of which are set 

 very wide apart (PI. CXLV, B; figs. 392, E, 449, B). 

 The well-worn canines measure only 25 millimeters 

 anteriorly and are proportionately the smallest we 

 have observed in any species of titano there. The 



