1010 The American Naturalist. [December, 
appears as a distinctive feature of the middle and upper 
Eocene Pachynolophus (Orohippus). The mesostyle was strongly 
developed in all the selenodont, buno-selenodont and lopho- 
selenodont types, such as the Artiodactyla and Meniscotherium, 
halicotherium, Paleosyops, the paleotheres and horses. Look 
at an upper molar of Merychippus and see what an important 
role these styles play (Fig. 1). First, we observe the “ para- 
style” and “mesostyle,” next most important is the “hypo- | 
style,” which develops near the hypocone upon the posterior 
cingulum of Mesohippus and Anchitheriwm and finally com- 
pletes the border of the 
“anterior fossette” or 
cementlake. The horse 
molar, by the way, furn- 
ishes the best illustra- 
tion of the value of trac- A 
ing back the various depoety 
portions of the crown 
to their birth-place in 27% \ 
the ‘primitive crown of 
Hyracotherium. Every 
turn in this labyrinth 
of folds is thus made 
perfectly clear.! 
A corresponding set 
of styles grows up on 
the lower molars, and 
it is very easy to locate ji) í 
them with reference to pariotitid pene e 
the reci Fig. 12.—THE HIGHEST DEVELOPMENT OF T 
if pa oe Baty ji Srvies es. A. Upper molar of Horses, Anchitherium 
and B. Merychippus. 
mind the fact that 
throughout the whole course of development the elements of 
each trigonid are placed just in front of those of the corre- 
sponding trigon; that is, the protoconid and metnoonid fit just 
parastyle mesostyle metastyle 
i 
i 
1 Mr. Lydekker has dirti called attention to the fact thiat i in the earlier 
study of. ymae the writer ESERIES the T terms n ed by 
Huxley; , i 
