64 THE MERYCOIDODONTID^ 



E. montanus is the only species with the infraorbital foramen above P 4 ; two others, E. condom and 

 E. major relktus, have this foramen over the interval between P 3 and P 4 ; all the other species 

 have it placed over P 3 . E. leptacanthus and E. socialis have dolichocephalic skulls and E. trigono- 

 cephalus is brachycephalic, but in the other characters they are normal for the genus. Leaving out 

 these three species, the average skull index of all the other species is 0.568; that for the John Day 

 forms is 0.558, and that for all the species east of the Rocky Mountains is 0.57, which are exceed- 

 ingly close for the whole group and for the two major sections whence these animals have come. 

 While the average skull index is 0.56, the various species, excepting the three mentioned above, 

 range from 0.50 to 0.63, which is nearly through the entire mesocephalic range. When we examine 

 the size of the bulla; we cannot find any real distinction other than specific. The bulla? are large 

 in some of the California, South Dakota, North Dakota, Nebraska, and Oregon species, and medium- 

 sized in other species from Colorado, South Dakota, Montana, Oregon, and Wyoming. The pos- 

 terior shape of the nasal bones shows that some of the species from Colorado, North and South 

 Dakota, Montana, and Oregon are pointed; others from Montana, Nebraska, Oregon, and Wyoming 

 are truncated. This means that approximately one-half of the species have posteriorly pointed nasal 

 bones and the other half have them truncated, regardless of the area where found, whether east or 

 west of the Rocky Mountains. 



The Marsh Collection shows that 84 per cent of the specimens of all species of Eporeodon in 

 the John Day Basin were found in the middle John Day and 16 per cent in the upper. The genus 

 Promerycochosrus laps downward into the middle John Day in about the same proportion, that is, 

 about 84 per cent in the upper portion and 16 per cent in the middle of the formation. This state- 

 ment of percentage applies of course only to the specimens in the Peabody Museum, and whether 

 or not it is true for all of the material in various museums I cannot say. 



Of 204 individuals of all species in the Marsh Collection from the John Day formation, a 

 quantitative study reveals that 13.2 per cent died while they still retained the milk dentition; 

 65.8 per cent died in the prime of life; 12.2 per cent lived past maturity; and only 8.8 per cent 

 reached old age. 



The majority of the Oregon specimens in the Yale Peabody Museum were collected at or near 

 Turtle Cove. The other areas, named in the order of their importance in yielding remains of this 

 genus, are as follows: Bridge Creek, Haystack Valley, five or six miles below Cottonwood, Clarno 

 Bottom, the North Fork, and Big Bottom. 



Etymology: Eporeodon (upon, near + Oreodon). 



Species: 



E. bulletins (Leidy) 1 869. 



E . condoni Thorpe 1921. 



E. dkkinsonensis (Douglass) 1907. 



E. helena? (Douglass) 1901. 



E. leptacanthus (Cope) 1884. 



E. longifrons (Cope) 1 884. 



E . longifrons perbullatus Thorpe 1921. 



E. major (Leidy) 1 854. 



E. major cedrensis Matthew 1901. 



E. major cheeki Schlaikjer 1934. 



E. major hybridus (Leidy) 1 869. 



E. major relktus Loomis 1924. 



E. montanus (Douglass) 1907. 



E. occidentalis (Marsh) 1873. Genotype. 



E. pacificus (Cope) 1884. 



E. parvus Thorpe 1 92 1 . 



E. socialis Marsh 1884. 



E. thurstoni Stock 1934. 



E. trigonocephalus (Cope) 1884. 



