332 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



cingula. There seems to be considerable range of 

 variation in the size of m^, as indicated below, so that 

 its large size in the type may not be truly specific. 



P. major, 32 by 38 to 33 by 40 millimeters. 



P. leidyi, 34 by 39 to 36 by 38 millimeters. 



P. robustus, 33 by 37 to 37 by 42 millimeters (type). 



P. granger!, 38 by 43 millimeters. 



P. copei, 34 by 40 millimeters. 



The size of the conules is also variable, as would be 

 expected in a character which was losing its hold in 



Figure 282. — Jaws and deciduous teeth of Palaeosyops leidyi? 



One-fourth natural size. Am. Mus. 1565; Bridger Basin, Wyo.; upper jaw. Am. 

 Mus. 12197; Henrys Fork Hill, Bridger Basin; Bridger formation, level C 2; lower 

 jaw. 



most titanotheres. The forward extension of the an- 

 terior part of the masseter and deepening of the malar 

 is a progressive metatrophy, as are also the closing 

 of the diastema (postcanine), the closure of the ex- 

 ternal auditory meatus, and the large size of the 

 skull. A very constant brachycephalic character is 

 the closing up of the postcanine diastemata, which 

 are either reduced or wanting The sagittal crest is 

 powerful but is differently 

 formed from that of the type 

 of P. leidyi, although not dis- 

 similar to that of other P 

 leidyi skulls. 



In addition to the specimens 

 which exactly or very closely 

 resemble Marsh's type, there 

 are others which appear to oc- 

 cupy an intermediate position 

 between P. robustus and the 

 older form P. leidyi of level C. 



Fluctuations. — The mesostyle and other premolar 

 characters (Pis. LVIII, LXII) in these intermediate 

 forms also show considerable fluctuation, but on the 

 whole there is a recognizable metatrophic advance 

 over P. leidyi. It appears that at each actual period 

 of geologic time Palaeosyops would show a consider- 

 able range of variation, partly individual, partly vari- 

 etal. For example, large size appears as an excep- 

 tional variation in a B level P. major (Am. Mus. 

 1.3116) and in the P. grangeri of Bridger C 1, while 

 small size of grinding series appears as an exceptional 



character in the high level (Bridger D 3) P. copei and 

 in the species of Palaeosyops from the Washakie Basin. 



Mutations. — Tliere seems to be considerable evi- 

 dence for the view that these "transitional" dentitions 

 and skulls bridge over the structural gap between 

 P. leidyi and P. robustus; indeed, it would appear 

 that this is clear. This view contradicts the idea 

 expressed elsewhere that P. leidyi "stands apart and 

 does not appear to form a connecting link between 

 P. major and P. robustus"; but that statement applies 

 only to the broadened occiput of the type of P. leidyi, 

 and since other skulls with narrow crests make up 

 the bulk of the species P. leidyi, too much should not 

 be made of the exceptional condition in the type. 

 It may well be that in one or two trifling characters 

 P. robustus may be shown ultimately to be descended 

 not from the true race of P. leidyi which lived at 

 Henrys Fork Hill during Bridger C 3 time but from 

 some other race of P. leidyi living to the north and 

 perhaps during Bridger C 1 to 3 time. However, by 

 such hairsplitting we obscure the grand evolution 

 lesson that P. major, leidyi, and robustus form suc- 

 cessive mutations which are very nearly if not quite 

 in a direct line, which might perhaps have been desig- 

 nated by trinomial names such as P. paludosus 

 paludosus, P. paludosus major, P. paludosus leidyi, 

 and P. paludosus robustus. 



Less progressive mutations, transitional from the P. 

 leidyi stage. — Some of the less progressive forms 

 are so much more primitive than the typical P. 



Figure 283. — Deciduous cheek teeth of Palaeosyops leidyi? 



Am. Mus. 1565. Natural size. The identity of mi, m' is positively established by comparison with the adult dentition. 

 The deciduous molars dp^, dp', dp*, are more molariform than the teeth which succeed them, p2, ps, p<. In this speci- 

 men the alveolus for the permanent canine lies closely appressed to p^ while p> has been forced out into association 

 with the milk molars dp^, dp', dp*. Thus in this middle Eocene titanothere the relations of the milk and permanent 

 teeth are the same as in the Oligocene titanotheres. 



robustus that they might be placed with equal exactness 

 in P. leidyi. They are especially interesting biologi- 

 cally in demonstrating the gradual inception of such 

 specific characters as are seen in Palaeosyops robustus 

 rather than the sudden saltation of this species out 

 of its predecessors. In comparing the following four 

 mutations we note especially the very gradual evolu- 

 tion of the rectigradations — namely, of the premolar 

 mesostyles — also the gradual atrophy of the conules. 

 First mutation: Of these the least progressive is 

 Am. Mus. 1586, consisting of a palate with full denti- 



