1887] Mesozoic and Cenozoic of North America. 451 
the Laramie in the absence of the numerous Dinosauria of the 
latter, and the presence of numerous Placental Mammalia in the 
former. On these grounds I at first referred the formation to 
the Cznozoic series, but further reflection induced me to place 
it as now arranged. e reason is as follows: Although Pla- 
cental Mammalia are not now known otherwise from Mesozoic 
beds, the, other forms of the Puerco are especially Mesozoic in 
character. Such are the Choristodere Reptilia and the Multitu- 
berculate Marsupialia, neither of which occur above, while both 
occur below the Puerco, the Multituberculata down to the 
Trias inclusive. Then the Placentialia are entirely peculiar in the 
_ absence of the Diplarthra and of the Rodentia,» orders always 
found in the Cznozoic beds. Then the characters of the Con- 
dylarthra and Amblypoda and many of the Creodonta, which 
represent Tertiary types, are so peculiar that we are led to sus- 
pect that when the Cretacic Mammalia are fully known they 
cannot differ very widely from those of the Puerco. 
But one area of this formation is definitely known; this is in 
Northwestern New Mexico and Southwestern Colorado. It 
consists of sandstones and soapy marls, and has a thickness of 
eight hundred and fifty feet. It is immediately overlaid by the 
Wasatch Eocene, and rests on the Laramie. 
TOTAL THICKNESS of the Mesozoic system (greatest) : 
Feet. 
1 I 6,000 
Turassic 6,000 
Cretacic 4, 
steretacic. 5,8 50 
31,750 
CÆNOZOIC REALM. 
This Realm is distinguished from the Cretacic, as well as from 
the Mesozoic formations as a whole, in North America by the 
=, following peculiarities. In Vertebrata: 
By the prenes of ZATON Bank 
"i of Roden 
“ "a of Weikitópiik Fishes. 
ád ‘a of Plectospondylous Fishes. 
“ _. of Osteoglossid “ 
“ “ee haryn h 
“ absence of ston Ne ees ap eG 
* «of Orthopod and Goniopod Dinosau: 
s "a AE aka Reptilia. 
* 
