524 THE MAMMALIA OF THE UINTA FORMATION. 



single internal lobe. The third has a single transverse crest, and the fourth has the 

 second crest just developing. In the true superior molars, the convex postero- 

 external lobe is slightly posterior to the junction of the posterior transverse crest 

 (metaloph), and there is a prominent cingulum embracing this lobe. In IsectolopJius 

 latidens of the Bridger, which is distinguished from Systemodon by the double in- 

 tei-nal lobes of pm.' and pm.^, the postero-external lobe is also slightly posterior to 

 the crest, but in the Pi'inceton specimen (/. latidens) there is no cingulum. Both 

 these forms have slight diastemata in the dental series. In I. annectens the same lobe, 

 which is homologous throughout w^ith the primitive metacone of the tritubercular 

 molar, is greatly produced beyond the junction of the posterior crest. The external 

 cingulum reappears, so that in this respect, the genus reverts to Systemodon, but pre- 

 sents such a marked advance even upon the I. latidens molars in other respects that 

 it must be considered a much more recent type. The third and fourth premolars 

 probably show a more distinctly developed pair of internal lobes than in I. latidens. 

 The inferior molars present the tapirine buttresses in the valleys and inflection of the 

 anterior crests which ai-e wanting in the Bridger species, but are occasionally seen in 

 the Bridger varieties of Ilyrachyus. Another developmental feature is the steady 

 growth of the anterior accessory tubercle, which is so marked in the tapir. The 

 comparative measurements of the teeth in these thi-ee genei-a show that, with the 

 development of the molar pattern and metamorphosis in the premolars, there was a 

 steady increase in size, extending proportionally in all parts of the dentition. 



It thus appears that the primitive tapirs were well distinguished from the other 

 lophiodonts very early in the Eocene period, and the tooth and foot structures charac- 

 teristic of the recent forms were acquired even more rapidly than in the parallel line 

 of primitive horses. 



The Miocene successor of IsectoJoplms is undoubtedly represented by the single 

 three-lobed lower molar from the White Kiver beds, which Dr. Leidy has referred to 

 Lopldodon occidentalis. By analogy with the premolar evolution in all other perisso- 

 dactyls we may anticipate that this tapir will be found to have three premolars like 

 the molars {Mesotajyirus) . 



TUIPLOFUS Cope. 



Syn. : Prothyracodon, nobis, Proc. Am. Phil. Soc, Sept. 2, 1887, p. 2G0. 



This genus was discovered by Prof. Cope in the Washakie or Upper Bridger 

 strata, and we find that it is very abundant in the overlying Uinta. The mandibular 

 dentition so closely resembles that of the smaller species of Ilyrachyus described 



