ENDOCRANIAL ANATOMY OF FOSSIL MAMMALS 367 
of which there are five on each side, are covered with a rich net- 
work of rami of the arteria ethmoidalis supplying the mucosa 
sinus paranasali. Judging from the condition in Merycochoerus, 
the paired median sacculations of the sinus frontalis lay im- 
mediately superior to the olfactory bulbs, and we have these 
bulbs exposed in Oreodon gracilis because the sinuses have been 
broken away. | 
Merycochoerus. In his discussion of the literature on endo- 
cranial casts Black (’20) called attention to errors in a previous 
figure of this endocranial cast (fig. 17) which was briefly referred 
to (Moodie, ’16) in conjunction with a discussion of the preserva- 
tion of casts of the paranasal sinuses. ‘The terminology has been 
revised in accordance with the suggestions of Black (20) and 
a new figure showing more carefully the modeling of the brain 
is given (fig. 17). The sulci and other cerebral markings are 
very similar to those of Oreodon. The dural venous sinuses are 
likewise fully as prominent. The comparative sizes of the 
cerebrum and cerebellum are the same in the two forms. Since 
Merycochoerus is a larger animal than Oreodon, the cerebrum 
is correspondingly larger, measuring 50 mm. in length by 48 mm. 
in greatest width. This cast displaces 77 cc. of water. 
Mesohippus. An incomplete cast from South Dakota (fig. 24) 
represents this form. Cope has figured the brain cast of a related 
early horse, Merychippus isonesus from the Deep River beds of 
Montana. These figures with many others were issued in a 
quarto volume under the supervision of Dr. W. D. Matthew (15) 
by the Americal Museum of Natural History. The brain cast 
is poorly figured, and since it is readily accessible, being no. 8105 
of the Cope Collection in the American Museum, should be 
restudied with additional material. The incomplete cast referred 
to Mesohippus (fig. 24) is in the collections of the University of 
Kansas. Since the endocranial anatomy of the early horses has 
been incompletely studied, this material will form a basis for 
such a study. 
Osborn (’88) has described the brain cast of Mesohippus, and 
Scott (’91) has discussed the brain case of this genus, comparing 
it to the modern horse. Since the present cast (fig. 24) adds 
