STRUCTURE AND AFFINITIES OF THE BRONTOTHERIDA. a 
The general characters of the Brontotheride are fully shown in 
a large series of specimens in the Yale College Museum. -The 
cranial structure of Brontotherium, the type genus, is well illus- 
trated in the nearly perfect skull of B. ingens Marsh, figured in 
plates i and ii. The only other genus of the group known with 
certainty is Titanotherium of Leidy (Menodus Pomel) which, ac- 
cording to the descriptions of that author, differed essentially in 
having four lower premolars, and in the absence of a third tro- 
chanter on the femur. Less important differences are seen in the 
composition of the teeth, and in the diastema between the upper 
canine and first premolar. 
The skull in Brontotherium is elongated, and resembles in its 
general features that of Rhinoceros. The occipital region is 
greatly elevated, and deeply concave posteriorly. The brain cavity 
is unusually contracted. ‘The top of the skull is concave longitu- 
dinally, and convex transyersely (plates i and ii). The zygo- 
matic arches are massive, and much expanded. The orbit is-small, 
and continuous with the elongated temporal fossa. ‘The nasal 
bones are greatly developed, and firmly coossified. -They support 
entirely, or nearly so, the large divergent horn-cores. Their ante- 
rior extremities are produced, and overhang the large narial orifice., 
The premaxillaries are diminutive, and do not extend forward 
beyond the end of the nasals. The palate is deeply arched above, 
especially between the premolars. The posterior nares extend 
forward nearly to-the front of the last molar. The Jachrymal — 
forms the anterior margin of the orbit. The malar extends for- 
ward beyond the lower margin of the orbit. The infra-orbital 
foramen is very large, and situated well forward. The zygomatic 
process of the squamosal is elevated, and incurved above. There 
_ is a massive post-glenoid process, and a large and somewhat shorter 
Paroccipital process (plate i). The post-tympanic process of the 
Squamosal is large, and quite external to the paroccipital process. 
The occipital condyles. are very large, and well separated. 
~ The mandible has a wide condyle, and a slender coronoid 
‘process. The angle is rounded, and slightly produced downward, 
a The symphysis is depressed, e AERES, very shallow in i frois, and 
~ Completely ossified. a 
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