ON SOME NEW AND LITTI.E KNOWN CREODONTS. 
1(55 
Together AAdth an exceedingly small brain, and a primitive type of dentition and 
carpus, we find associated a highly differentiated metacarpus and tarsus, a reduction 
of digits like that of modern carnivores, and finally a mode of articulation in the 
lumbar and posterior dorsal vertebrae more perfect than in any existing carnivore. I 
cannot confirm Professor Cope’s and Mr. Wortman’s suggestion of a relationship 
between Mesonyx and the seals. A certain similarity in the claws and teeth is 
undoubtedly present, but the characters of the skull, brain, vertebra? and limbs are 
altogether different. The ancestry of the seals is more probably to be looked for in 
the Bridger genus Megenceplicdon. Altogether, then, it would seem that Mesonyx and 
Paclvymno, form highly specialized side branches of the Creodonta which died out 
without successors. No living animal seems to stand in direct relationship with these 
extinct genera. 
History. — The genera Mesonyx and Synoplothermm were established by Profes- 
sor Cope in 1872 upon specimens from the Bridger Eocene, Pachycena in ISTC to 
receive certain species from the Wahsatch of New Mexico. Subsequently Professor 
Cope concluded that all three forms belonged to the same genus. But as we have 
seen, Mesonyx is very different from Pachycdna, and more perfect specimens wnll very 
probably show that Synoplotlierium is also distinct. But this question cannot be 
decided at present. 
Mesonyx, Cope 1872 (=1 Synop>lotherium., Cope 1872,) MesonycMdce with the den- 
tal formula: IH, C}:}, Pm. 4 :| Mfif^ 40; no diastemata behind canine; fore and hind 
limbs of nearly equal length, metapodials elongate, and animal probably digitigrade. 
Pachywna, Cope 1874. Dental formula; I|:|, CJj, Pm. tl, MU = 42;' dias- 
temata in front of and behind 1st premolar ; hind limbs much longer than fore limbs ; 
metapodials short, and animal probably plantigrade. 
Tlie family Mesonychidcz may be defined as: Creodonta with trochlear ankle 
joint ; “ molar teeth in both jaws consisting of conic tubercles and heels ; none 
sectorial ; a preglenoid crest.” (Cope.) 
So far as is at present known, Mesonyx seems to be confined to the Bridger and 
Uinta formations, and Pachycena to the Wahsatch. 
Measurements. 
Mesonyx 
OBTUSIDENS 
Z • 
§ H 
^ g 
tf 
W ^ 
Length of skull (along base) ........ 
M. 
.279 
M. 
.227 
“ cranium ......... 
.125 
.114 
“ face .......... 
.154 
.113 
Breadth across zygomatic arches ....... 
.165 
— 
“ of forehead ......... 
.080 
.075 
“ of cranium (greatest) ....... I 
.055 
.054 
