FORMATIONS OF THE NACIMIENTO GROUP 709 



notes of several years' residence and exploration by my correspondent, David 

 Baldwin, which connect those made by myself in New Mexico in 1874, published 

 in the Wheeler Survey report, with those made by Holmes and Endlich in 1878 in 

 Colorado, and published in the Hayden Survey report. 



At the locality where best developed, the Puerco beds have a thickness of 

 about 850 feet, and contain Mammalia to the base (see Naturalist for April and 

 May, 1885). The Laramie beds succeed downward, conformably it is thought 

 by Mr. Baldwin, and have a thickness of 2,000 feet at Animas City, New Mexico. 

 They rest on Fox Hills marine Cretaceous of less thickness. A few fossils sent 

 from time to time by Mr. Baldwin identify the Laramie. This is especially done 

 by the teeth of the dinosaurian genus Dysganus Cope, which is restricted to the 

 Laramie formation everywhere. Also by the presence of the genera Laelaps and 



Diclonius, which in like manner do not extend upward into the Puerco beds 



It is thus evident that the Puerco formation is quite distinct from the Laramie, 

 although it is possible that it may be proper to associate it with the Laramie in 

 the post-Cretaceous series. When the Cretaceous mammalian fauna comes to be 

 known, it will be very apt to agree with the Puerco in its leading features. These 

 are the absence of Perissodactyla and of Rodentia, and of course of mammalian 

 orders not found below the Miocenes; and in the constitution of the mammalian 

 fauna by Condylarthra, Bunotheria and Marsupialia exclusively. The post- 

 Cretaceous series as a whole may be ultimately distinguished from the Tertiary 

 by these peculiarities, together with the presence of the reptilian genus Champ- 

 sosaurusJ^ 



Concerning the taxonomy of the original Puerco, Cope was at 

 first inchned to place the beds in the Eocene because of their appar- 

 ent conformability at the top v^ith beds known to be of that age. 

 He freely referred to the "Puerco Eocene" in his earlier papers but 

 in 1885 receded from this opinion and, on the basis of the Mesozoic 

 affinities of the fauna, placed the Puerco in what C. A. White had 

 proposed to call the post-Cretaceous. This term has been used 

 more or less to the present time as signifying a zone between the known 

 Mesozoic and Cenozoic eras. But it is evident that Cope considered 

 the Puerco fauna more closely related to the Cretaceous than to the 

 Tertiary faunas as indicated in the following quotation from the 

 American Naturalist: 



The fauna of this horizon is well distinguished from that of the Laramie in 

 the absence of the numerous Dinosauria of the latter, and the presence of numer- 

 ous Placental Mammalia in the former. On these grounds I at first referred the 

 formation to the Cenozoic series, but further reflection induced me to place it as 

 now arranged. The reason is as follows: Although Placental 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 



