with Descriptions of New Genera and Species. 95 



that assumption offers an explanation of why the forms 

 from this area are different from those in the rest of the 

 basin. One of the future problems, and one which is 

 indeed critical, is a detailed geologic and paleontologic 

 study of the North Fork area. 



The number of skulls^ of John Day Eporeodons repre- 

 sented in the Marsh Collection is approximately 300, 

 while fragmentary materials indicate at least 50 more 

 individuals. On a basis of 218 well preserved skulls, a 

 quantitative study was made, some of the results of which 

 are briefly stated below. 



In determining the relative age of the individuals at 

 death, the computations were made on a basis of 10, as 

 follows : 0-3, milk dentition ; 4-6, average medium age 

 with 5 considered as mid-life, or fully adult; 7-8, those 

 with the tooth pattern of M^ obliterated and that of M- 

 much worn; and 8-10, those having the tooth pattern 

 of true molars very much worn or completely obliterated. 



Of 204 individuals in which the age was determined, 27 

 or 13.2 per cent died between 1 and 3 ; 134 or 65.6 per cent 

 between 4 and 6; 25 or 12.2 per cent between 7 and 8; 

 and 18 or 8.8 per cent between 8 and 10. This summary 

 for all species is typical for the individual species with 

 but few exceptions. Eporeodon occidentalis shows 23.8 

 per cent that died having milk dentition and no individ- 

 uals that reached an age between 8 and 10. Eporeodon 

 pacificus shows 18 per cent reaching a very old age, 8 and 

 above. In E. leptacanthus, 6.1 per cent died with 

 deciduous dentition and only 1.5 per cent reached the age 

 of 8, while the majority died in their prime, between 4 

 and 6. Half the specimens of E. trigonocephalus are 3 

 or below. 



The majority of the specimens were collected at or 

 near Turtle Cove. The other areas, named in order of 

 their importance in yielding remains of this genus, are as 

 follows: Bridge Creek, Haystack Valley, 5 or 6 miles 

 below Cottonwood, Clarno Bottom, the North Fork, and 

 Big Bottom. 



I am deeply appreciative of the privilege, afforded me 

 by Messrs. Matthew and Granger, of measuring' and 

 studying the Cope types of this genus in the American 



^ The present study is based wholly on skulls and jaws; the Marsh Collec- 

 tion, however, contains an abundance of skeletal material as well. 



