1881.] 



397 



[Cope. 



Fig. 2. Part of right maxillary bone of Meniscotherium chamense Cope, from 

 the Wasatch bed of New Mexico. From Report Capt. G. M. Wheeler, IV, ii, 

 PI. LXVI. 



MENODONTID^. 



The known genera of this family are not nuinerous. They are defined 

 as follows : 



I. Vs of inferior molars probably incomplete ; superior molars with in- 

 termediate tubercles. 



Internal cusps of superior molars well separated Acoessus. 



II. Inferior molars with the crowns thrown into two Vs ; superior 

 molars without intermediate tubercles. 



a. Last superior premolar only with two inner tubercles. 

 Incisors present Biplacodon. 



aa. All the superior premolars with two interior cusps. 



Six inferior incisors, canines very large DcBodon. 



Six inferior incisors ; canines very small Menodus. 



No inferior, and four small superior incisors ; canine very small 



Symhorodon. 

 The first appearance of this family was in the Early Eocene in the genus 

 Acoessus Cope, which was a cotemporary of HyracotTierium, and which it 

 resembles in some respects. Its typical species was called Hyracotherium 

 siderolitMcum by Pictet, its describer, but Kowalewsky has already ex- 

 pressed the opinion that the species does not belong to that genus. It is 

 from the Lower Eocene of Mauremont, Switzerland. The remaining 

 genera are, as yet, American, excepting one, which is represented by an 

 Austrian species, not yet well known. Biplacodon, in its simpler pre- 

 molars, approaches the ChaUcotTieviidm, and is the oldest of the American 

 genera. It is from the Uinta or Upper Eocene. Menodus and Symhorodon, 

 which include some species of gigantic size, belong in the White river or 

 Oligocene, while Dcpodon has so far only been obtained from the Truckee 

 or Upper Oligocene. 



