MARSUPIALIA. 113 



The characters of the fossre of the mandibular ramus are those of the 

 marsupial order, and much like those of the family of the kangaroos. The 

 absence of connection between the masseteric and pterygoid fossfe at the 

 dental foramen distinguishes Ptilodus from that famil}^ The differences in 

 the dentition have been already discussed. 



The announcement of the discovery of this genus in the Eocene forma- 

 tion was a circumstance of much interest, and it has shown how persistent 

 the type of the Plagiaidacidce has been. It is true that no representative of 

 the Plagiaulacidce has yet been obtained from the beds of the Cretaceous 

 period, which represent the long interval of time which elapsed between the 

 Jvxrassic and the Eocene. We will not, however, on this account permit the 

 supposition that they did not exist at that time. 



The existence of the Plagiaidacidce in the Eocene period v/as first ascer- 

 tained by Dr. Victor Lemoine, of Reims, France, and was announced to the 

 Geological Society of France at its meeting of January lO, 1881, and pub- 

 lished in its Bulletin for 1881, p. 1G8, for May. This announcement had 

 escaped my observation when six months later (October) I published the 

 account of its discovery in New Mexico. I, at that time, gave the genus 

 the name of Ptilodus, and I am not aware that Dr. Lemoine has yet printed 

 any name, either generic or specific, for the form discovered by him. 



Ptilodus medt^vus Cope. 



Aniericau Naturalist, ISKl, p. 9'21, November; (published October 28). 

 Plate XXIII c; fig. 1 



This species is represented by two mandibular rami of probably the 

 same animal, one of which lacks the part anterior to the premolars, and the 

 other the part posterior to the premolars; by a single fourth premolar tooth 

 of a second individual; and by all the inferior molars of a fourth. 



The ramus is short, and is deep posteriorly ; anteriorly its depth is re- 

 duced by the concavity of the surface at the diastema, as in most rodents. 

 The inferior border is i-ather thick anterior to its posterior expansion. The 

 symphysis is not coossified. The incisor tooth is quite slender, and its sec- 

 tion is a vertical oval, a little flattened on the inner side. The enamel band 

 covers less than two-fifths of the face of the tooth, beginning with the inner 



