M. R. Thorpe — Arceocyon. 371 



Art. XXXYI — Arceocyon, a Probable Old World Migrant; 

 by Malcolm Rutherford Thorpe. 



[Contributions from the Othniel Charles Marsh Publication Fund, Peabody 

 Museum, Yale University, New Haven, Conn.] 



In this Journal for June, 1921, I described the lower 

 jaw of a carnivore, collected through the efforts of Pro- 

 fessor Marsh, in Oregon, in 1874. For this carnivore I 

 proposed the name Pliocyon marshi. Subsequently, I 

 substituted the name Arceocyon to supplant my preoccu- 

 pied Pliocyon, and a note to this effect was printed in the 

 January, 1922, issue of this same journal. 



The presence of this lower jaw in the New World is 

 decidedly interesting, for to my knowledge no specimen 

 comparable to it has been described or reported from 

 among the ancient fauna of North America. 



There are but two similar forms in the extinct world 

 faunas with which we can make close comparison, and 

 one of these is much closer than the other. The nearest 

 ally of our American form is Simocyon primigenius Roth 

 and Wagner, from Greece. This genus and species is 

 probably somewhat more specialized, although it was pre- 

 served in strata of older geologic age than was the Oregon 

 fossil. The two specimens show a curious admixture of 

 specialized and generalized characters, and it is rather 

 difficult to be certain which represents the later phase of 

 evolutionary development. For example, Arceocyon has 

 two incisors (specialized), Simocyon, three, while the 

 former has two premolars and the latter but one (special- 

 ized). In other characters the Grecian genus is larger 

 and differently proportioned. 



The other European species of Simocyon is S. dia- 

 phorns Kaup, from Germany, and it very materially dif- 

 fers from the Grecian and American forms. In compar- 

 ison with the Oregon specimen, Kaup's type possesses 

 P 2 and P 3 (lacking in both of the other specimens under 

 consideration) ; it has a larger and higher metaconid on 

 Mi, and a smaller hypoconid; it has a shorter and lower 

 P 4 , with more prominent basal heel; M 2 is set in the 

 ascending ramus, while in A. marshi it is nearly level 

 with the tooth-row; and there are still other characters 

 which differentiate it from both of the other forms. 



