Tooth-Structure — Lamhe. 245 
metaloph, as well as by other characters. M. celer t Marsh 
and M. itiontanensisX Osborn are two other nearly related 
but distinct species from the Lower Oligocene. The pres- 
ence of the internal cingulum is the most interesting char- 
acter in the dentition of M. westoni. This character to- 
gether with the absence of a hypostyle points to this species 
being probably the most primitive of the known horses of 
Oligocene age. 
No teeth from the lower jaw were obtained during the 
expedition of last summer. Figures of the two lower 
. molars, discovered in 1884, are given in Cope's memoir of 
1 891. 
Measurments oi Upper Molar obtained in 1904. 
Transverse diameter 013 
Antero-posterior diameter 0102 
Hight of protocone 0045 
Hight of hypocone 0045 
Hight of ectoloph 0062 
Explanation of figures. 
Figure 1. Inferior view of crown of upper molar of Mesobippus 
westoni. 
Figure 2. Exterior aspect of the same. 
Figure 3. The same viewed from within. 
Figure 4. Anterior view of the same. 
All the figures are twice the natural size.: pa. paracone; me 
metacone; pr. protocone; by. hypocone; pi. protoconule; ml. meta- 
conule; ps. parastyle; ms. mesostyle; mts. metastyle. 
EDITORIAL COMMENT. 
Summer Courses in Field Geology, 
The joint announcement of summer field courses in 
geology from which some extracts are given on another 
page, presents the opportunity for the field study of geo- 
logy under experienced instructors in various parts of the 
country for the coming season, in the form most convenient 
for the inspection of teachers and students at other institu- 
tions than those which offer the courses. Courses given 
t American Joutnal of Science. (3), vol. vii, 1874, Notice of new equine 
mammals from the Tertiary formation, p. 251. 
t 1904, Op. cit. 
