1899-] Wortnian, Restoration of Oxyccna lupiiia. 14/ 



author, the same cusp reduction and simplification is observed 

 in this succession as occurs in the Dissacus-Pachycena-Mesonyx 

 series. 



The second subfamily, Mesonychinas, so far as at present 

 known, finds its oldest representatives in the Torrejon beds in the 

 single genus Dissacus, with two well marked species. It is, in- 

 deed, very doubtful, if not impossible, that any of the known 

 species of Triisodon can be placed ancestral to the present genus, 

 although it is not at all improbable that some slender jawed type 

 in the Puerco having the cusp pattern of Triisodon will be found 

 to have commenced an early modification of the teeth terminating 

 in Dissacus. The evidence for the view that the inferior molar 

 pattern in this genus is a degenerative one, is found in the fact 

 that in the succeeding Wasatch Pac/iycena the postero-internal 

 cusp of the trigon is still more reduced than it is in Dissacus, 

 while in the Bridger Mcsonyx it exists as the merest vestige, 

 the cusps of all the molars at the same time assuming a very 

 rounded and characteristic conical form. 



Pachyaena intermedia, sp. nov. 



It has been shown by Osborne and Earle ' that neither of the 

 known species of Pachycena {gigantea and ossifraga) can stand 

 directly in the line of descent leading to Mesonyx on account 

 of the greater reduction of the last upper molar in the two species 

 of Dissacus {tiavajovius and saurogiiathus) than in Pachycena. 

 Scott has shown ^ that Mesonyx has only two superior true molars, 

 so that any species of PachycBna which exhibits a less reduction of 

 the last upper teeth than Dissacus cannot be placed ancestral 

 to Mesonyx. Fortunately the collections of the Museum contain 

 a specimen of a Pachycena from the Big Horn, obtained by the 

 expedition of 1896, which shows, proportionately, as great or a 

 little greater reduction of the last upper molar than Dissacus 

 navajovius. The specimen consists of the last two upper molars 

 and bears the Museum number 2854. The teeth in question are 

 somewhat smaller than the corresponding ones in P. ossifraga 

 and exhibit a considerably greater reduction of the last molar in 

 comparison with the tooth in advance than in this species. It 



* 'Fossil Mammals of the Puerco,' Bull. Amer. Mus. Nat. Hist., Vol. VII, March. i8q5, p. 39, 

 £ ' Some New and Little Known Creodonts,' Jour. Philad. Acad. Nat. Sci., 1886, Vol. I. 



