M, R. Thorpe — A New Merycoidodon, 335 



putations were made on a decimal basis of to 10, as in 

 the species of the genus Eporeodon of the John Day 

 basin, Oregon. The same method of procedure was 

 followed : 0-3, deciduous dentition ; 4-6, average medium 

 age with 5 considered as mid-life and fully adult; 7-8, 

 those with the tooth pattern of the triturating surface of 

 M^ obliterated and that of M^ much worn ; and 8-10, those 

 having the tooth pattern of all the true molars very much 

 worn or completely obliterated. Of 210 individuals, 44, 

 or 20.9 per cent, died between 1 and 3 ; 138, or 65.7 per 

 cent, between 4 and 6; 21 or 10 per cent, between 7 

 and 8; and 7, or 3.3 per cent, between 8 and 10. This 

 summary for M. cidhertsonii is typical, in the main, for 

 the other species, although they are not represented by so 

 many individuals, and therefore the data derived from 

 them are not so reliable. 



In comparison with the figures resulting from a similar 

 study of the John Day Eporeodons, we find a remarkable 

 agreement in the percentage of those that died between 

 4 and 6, both groups showing 65 per cent. In the other 

 age classes, the figures would seem to indicate that the 

 infant mortality in the White River area was greater than 

 in the John Day basin where it was seen to be 13.2 per 

 cent, and that more individuals reached an older age in 

 that basin, the classes showing 12.2 per cent for post- 

 mature and 8.8 per cent for extreme old age. 



These two groups are widely separated in time and 

 geographic locality, and undoubtedly their environmental 

 conditions were considerably different. 



Description of Species. 

 Merycoidodon culhertsonii culhertsonii (Leidy) 1848. 



Specimens of this form were collected in Colorado, 

 South Dakota, Wyoming, and Nebraska, fully 50 per cent 

 or more being obtained in the latter state. Nearly all 

 parts of the skeleton are fully represented in the Yale 

 collection, as well as several very excellent endocranial 

 and endofacial casts. 



A few specimens show peculiarities worthy of note. 

 Cat. No. 12477, Y. P. M., from Scott's Bluff, Nebraska, 

 consists of both rami, showing each P4 placed obliquely 

 forward, and the premolar series are unusually short. 



