EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



321 



tion, level B, has the same general characters but is of 

 smaller size and unfortunately lacks the teeth. 



In a third, rather young jaw (Am. Mus. 11692) from 

 Grizzly Buttes, level B 2, the molars are only a shade 

 smaller than those of the type, with which they agree 

 in most details; but this younger jaw is slightly more 

 progressive in the structure of p4, of the paraconids, 

 and of the metastylid folds in mi-ma. 



In a fourth jaw (Am. Mus. 13118), from B 2, the 

 characters and measurements agree perfectly with 

 those of the type and neotype. 



In a fifth jaw (Am. Mus. 12679), from B 3, the dental 

 measurements are slightly smaller than in the neotype. 



The finely preserved upper teeth (Am. Mus. 13032) 

 fit exactly with the neotype jaw. These upper teeth 

 represent one of the oldest (level B 1) and certainly 

 most primitive Bridger titanotheres known. They 

 differ from the type of L. priscus in the following re- 

 spects: 



1. They are from a lower level, very low in B 1 

 (Granger). 



2. They are larger. 



3. The premolars are far more primitive — the most 

 primitive known, in fact, among Bridger titanotheres. 



4. The hypocone-cingule on m^ is imperfect. 



They agree well with the neotype lower jaw of P. 

 paludosus in the following respects : 



1 . They are from the same general level (B 1 ) . 



2. They correspond in general size. 



3. They show exact fitting of upper teeth (Am. 

 Mus. 13032) with lower teeth (neotype of P. paludosus, 

 Am. Mus. 11680) — that is, certain measurements be- 

 tween cusps in the upper jaw agree with corresponding 

 measurements between interspaces and valleys of the 

 lower teeth. The accuracy of this correspondence is 

 highly significant. 



4. They show correspondingly backward develop- 

 ment in the upper premolars of No. 13032 and of the 

 lower premolars in the neotype of P. paludosus. 



5. The canines are large and rounded and are charac- 

 teristic of the genus Palaeosyops. 



In short, this specimen. No. 13032, appears to fill the 

 great want of an upper dentition of P. paludosus. The 

 presence of a cingule-hypocone on m^ does not neces- 

 sarily excluded it from Palaeosyops, because this cusp 

 is more or less variable, a fact shown by its absence 

 in L. monoconus and its presence in P. diaconus ( = 

 rolustus). 



Specific cTiaracters of the neotype and other referred 

 specimens. — Although the first of the Eocene titano- 

 theres to be discovered and constituting the classic 

 type of the genus Palaeosyops, and also the oldest in 

 point of evolution, this primitive species is still im- 

 perfectly known because of the rarity of the speci- 

 mens on this low geologic level. A vast amount of 

 confusion has attended the previous description of P. 

 paludosus. We are now for the first time enabled to 

 characterize it sharply as a stage in which the second 



and third superior and inferior premolars are ex- 

 tremely simple in point of cusp evolution. Referring 

 the reader to the previous systematic discussion in 

 Chapter III, we may here summarize our knowledge 

 of the neotype and the referred specimens. The 

 lower jaw (Am. Mus. 11680) is taken as the neotype, in 

 which m2 agrees exactly with that of the type. This 

 jaw exhibits the following specific characters: P2-m3, 

 151 millimeters; p2 extremely simple, with faint trace 

 of paraconid fold and noncrescentic hypoconid; ps 

 with rudimentary paraconid, metastylid fold, and 

 crescentic hypoconid ; p4 with very decided paraconid, 

 elevated metaconid, distinct metastylid fold, broadly 

 crescentic hypoconid, extremely rudimentary ento- 

 conid; m2 closely agreeing in form and measurement 

 with that of the type; ms with a narrow, subcrescentic 

 hypoconulid, median in position. 



The following measurements of two specimens re- 

 ferred to Palaeosyops paludosus should be compared 

 with the table of measurements on page 316: 



Measurements of Palaeosyops paludosus 



Upper teeth, Am. Mus. 13032, Bridger B 1: Millimeters 



Pi-m3 102 



P2-m3 144 



P>-p< 71 



M>-m3 : 91 



PS ap. by tr 18X22 



Ml, ap. by tr 25X26 



MS, ap. by tr 32X34 



Neotype lower jaw, Am. Mus. 11680, Bridger B 1: 



Condyle to symphysis (estimated) 340 



Length of symphysis (estimated) 85 



Depth of ramus behind ms 85 



Condyle to angle 165 



Pi-m3 (estimated) 169 



Mi-ms 98 



PS ap. by tr 17X12 



Ml, ap. by tr 26X18 



MS ap. by tr 43X23 



Palaeosyops major Leidy 



Plates LVIII, LXII; text figures 89, 215, 268 C, 272, 279, 515, 

 516, 533-535, 546, 550, 686, 721, 741 



[For original description and type reference see p. 158. For skeletal characters see 

 p. 620] 



Type locality and geologic horizon. — Bridger Basin, 

 Wyo. ; Bridger formation, levels B 2 to 4, Palaeosyops 

 paludosus-Orohippus zone Leidy's type jaw is simply 

 recorded from Grizzly Buttes, equivalent to Bridger 

 B 2. The geologic range of this species, as exposed in 

 the upper portions of Grizzly Buttes and the lower 

 portions of the Cottonwood Creek section, covers 

 Bridger B 2, B 3, and probably B 4. 



Specific characters. — Of intermediate to large size; 

 total length of skull (estimated), 389 to 436 millime- 

 ters; p2-m3, 164; p^-m', 147; lower premolars somewhat 

 more progressive; superior premolars without meso- 

 styles; p^ with two external cones — -that is, both pro- 

 tocone and tritocone. No rudiments of osseous horns. 

 Cephalic index 74 to 77. 



