742 THE AMYZON BEDS. 



A dorsal vertebra, probably posterior, has a dej^ressed centrum nearly- 

 square in outline viewed from below, with lateral concavities and obtuse 

 median ridge. The articular surfaces are slightly opisthoccelous. A lumbar 

 shows subcylindric postzygapophyses as in Mesonyx. A femur displays a 

 well-developed third trochanter. The rotula is strongly elevated, much 

 more so than in the species of Mesonyx. Immediately above it, the section of 

 the shaft is subtriangular, the wide base posterior. 



Measurements. 



M. 



{ vertical 008 



Diameters of centram of a dorsal vertebra } trausverse 015 



(anteroposterior _. ... .017 



Length of a lumbar vertebra 024 



Diameters of shaft of femur below third trochanter^ nnteroposterior Oil 



( transverse 017 



Diameters at condyles ^ ''°*'""°P''«t^"'"^ 030 



transverse 028 



adyles < 



SUPPLEMENT TO PART FIRST 

 THE AMYZON SHALES. 



In the uncertainty whether this formation belongs to the Eocene or 

 Miocene series, I introduce it between the two. 



In the American Naturalist for May, 1879, I named the strata of this 

 epoch, that of the Amyzon beds, from the cliaracteristic genus which it 

 includes, and referred it to the later Eocene or early Miocene eras. Its 

 fauna includes twelve species of tishes, distributed as follows: Trichophanes 

 Cope, 3 sp.; Amyzon Cope, 4 sp. ; Rldneastes Cope, 1 sp.; AmiaL., 2 sp.; 

 and two of birds; {Charadrius and PalcBOspiza). These genera have not 

 been found represented in the fish fauna preserved in the Green River 

 shales, which embraces eight genera and twenty-four species. But they 

 occur in several species and specimens in the South Park of the Rocky 

 Mountains of Colorado, associated with the genera Bhineastes and Amia, 

 neither of which has yet been found in the Green River formation. The 

 first named is common in the Bridger, but in a different form, and the 

 generic identity is not yet fully established. The Amia is represented in 

 the Bridger by Pappiclithys, but in the former genus the characteristic parts 

 have not yet been seen in the South Park specimens, so that here also the 



