282 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBjRASKA 



as correct, it seems impracticable to divide Lamb- 

 dotherium into a large series of species, although 

 the development of the 

 premolar cusps certainly 

 warrants specific separa- 

 tion. 



These extreme stages are 

 therefore grouped together 

 for the present as muta- 

 tions between Lambdothe- 

 rium priscum, represented 

 by the simplest type (No. 

 12822), and Lambdotherium 

 progressum, represented by 

 the most advanced type 

 (No. 14917). Halfway 

 between the extremes is 

 the type species L. popo- 

 agicum. 



The cusp evolution is very 

 interesting, including the 

 following elements: Ante- 

 rior lobe — protoconid (pr*), 

 paraconid (pa*^), metaconid 

 (me"*), metastylid (ms"^); 

 posterior lobe — hypoconid 

 (h**), entoconid (en''). 



The series represented in 

 Figure 234, including L. 

 priscum at the bottom and 

 L. progressum at the top, 

 presents a complete mor- 

 phologic transition or epit- 

 ome of premolar evolution, 

 it being essential to note 

 that we do not know 

 Figure 234.— Lower premo- whether this corresponds 

 lars of three "species" or .,•■ i 



f T<7,fh o T-ooi succession m 



mutations of Lambdothe- 

 rium, illustrating progres- 



with 

 time. 



real 

 At the base p2 and 



sive evolution of the pre- p^ are excessively simple, 



NatoalsL^'selnd, third, and fourth but at the Summit Pa haS 

 lower premolars of the left side, inner a large paraCOnid and mcta- 

 sideview. A-E, i. priscum; F-H, i. . , , , 

 popoagimm; I-L, i. progressum. AH COnid, and P4 haS a para- 

 American Museum specimens from „ -J rnptflponid metfl- 

 the Wind River Basin. A, No. 12822 COUIQ, metaCOniQ, meia- 



stylid, and rudiment of an 

 entoconid, which is devel- 

 oped as a distinct cusp in 

 certain specimens (such as 



(type), 3 miles east of Lost Cabin; B, 

 No. 14916, Alkali Creek, Buck Spring; 



C, No. 14900, Alkali Creek, Wolton; 



D, No. 14912, Dry Muddy Creek, 18 

 miles above mouth; E, No. 14914, 

 Alkali Creek, Wolton; F, No. 14915, 

 Alkali Creek, Wolton; G, No. 14924, 

 Dry Muddy Creek, 18 mUes above Am. MuS. 14924). 

 mouth; H, No. 4863 (type). Wind 7-7 



River valley; I, No. 14913, Muddy PremoiaTS relatively prO- 



Creek, south side, 18 miles above • tj_ • 



mouth; J, No. 14919, Alkali Creek, gressive.—lt IS vcry impor- 



Wolton; K, No. 14918, lower Alkali font in nntA tViot n nnrl 

 Creek; L, No. 14917 (type). Alkali ^^^^ ^^ ^^^^ ^'^^^ Ps ^^^ 



Creek, Buck Spring. p^ in the progressive forms 



are much more advanced in evolution than the 

 corresponding teeth in the contemporary Eotitanops; 



in fact, p4 is submolariform and lacks only the prom- 

 inence of the metaconid to be like a molar. P2 in 

 Lambdotherium is almost as progressive as ps in 

 Eotitanops. Even in the middle Eocene species 

 Palaeosyops leidyi, ps, p4 are not so far advanced as 

 in Lambdotherium. We observe also another dis- 

 tinctive character: Whereas in Palaeosyops the meta- 

 conid arises as a bud or reduplication of the proto- 

 conid, in Lambdotherium it springs from the posterior 

 side of the protoconid. These details are of impor- 

 tance as demonstrating the accelerated rate of evolu- 

 tion of the premolar cusps as a character of lamb- 

 do theres. 



Superior premolars. — Comparatively few well-pre- 

 served superior premolar series are Icnown, so it can 

 not be determined whether there is a corresponding 

 series of mutations in the evolution of the upper teeth. 

 In the specimens Am. Mus. 14902, 14900, 14911, 

 14907 the following characters are observed: (1) p^ 

 very simple, with single external protocone and rudi- 

 ments of the deuterocone and sometimes of the 

 tritocone; (2) p', p* with rudimentary parastyle, 

 deuterocone, tritocone, rudimentary crests connecting 

 deuterocone with protocone and tritocone, respec- 

 tively, faint conules sometimes observed on these 

 crests. 



A series of deciduous premolars (Am. Mus. 14934) 

 exhibits dp^ somewhat more complex than p^, dp^ 

 elongate, quadricuspidate, with prominent parastyle 

 and mesostyle. 



Molars. — The inferior molars are highly character- 

 istic teeth, distinguished especially by the elevation of 

 their crescents; the protolophid consists of an elevated 

 protoconid, metaconid, and metastylid, or double 

 internal cusp, which is very distinct in unworn teeth. 

 This reduplicate cusp, which develops in the upper 

 Eocene species of horses and also in the true chali- 

 cotheres, is not present in the titanotheres of the 

 middle Eocene, such as Palaeosyops. A rudimentary 

 hypoconuHd is usually observed in nii and m2 and 

 develops into a strong crescentic third or posterior 

 lobe in 1113. 



The superior molars, as observed in five specimens 

 in the American Museum (Nos. 14900, 14902, 14904, 

 14907, 14911), are fairly uniform in character, with 

 very prominent parastyles, mesostyles, variable pro- 

 toconules, rudimentary or lophoid metaconules. (PI. 

 LIV, A, B; fig. 235, A.) A very distinctive feature is 

 the large hypocone on m^ The following characters 

 should also be noted: (1) The transverse diameter 

 always exceeds the anteroposterior; (2) m' is a rela- 

 tively small tooth; m^ is usually the largest tooth of 

 the series; m' is usually intermediate in size but some- 

 times is the largest tooth of the series. 



