0. C. Marsh — Principal Characters of ike Brontotheridce. 339 



Diastema behind upper canines. Inner basal ridge on upper 

 premolars not continuous. Nasals more elongated. A postor- 

 bital process. Third trochanter rudimentary or wanting. Type 

 Megacerops Coloradensis Leidy, 



3, Brontotherium Marsh, {Symborodon Cope, in part) {Mio- 

 hasileus Cope.) 



Dentition =InciBors— ; canines— ; premolars— ; molars—. 



No superior diastema. Strong continuous basal ridge on inner 

 side of upper premolars. No postorbital process. Third tro- 

 chanter distinct. Type B. gigas Marsh. 



4. Diconodon Marsh {Anisacodon). 



Dentition r=:Incisors— ; canines— ; premolars— ; molars—. 



No superior diastema. Strong inner basal ridge on upper pre- 

 molars. Last upper molar with two inner cones. No postor- 

 ital process. Type D. montamis Marsh. 



In the dentition and skeleton, the BrmitoHieridue more nearly 

 resemble the Eocene Diplacodon, than any other American 

 genus, and they may yet prove to be nearly related. The ani- 

 mals of that genus were of much smaller size, and entirely 

 without horns. The relations of the Brontotheridce. to the genus 

 Chalicoiherium Kaup, cannot at present be determined. 



In comparing the Brontotherida with the equally gigantic 

 Dmocerata of the Eocene, several striking points of resemblance 

 will be at once noticed ; especially the presence of horn-cores 

 in transverse pairs : the general structure of the limbs ; and the 

 short and thick toes. The differences, however, between these 

 two groups are still more marked. In the Brmitotheridm there 

 IS but a single pair of hom-cores, and no crest around the ver- 

 tex. The structure and number of the teeth are quite different, 

 while the small canines and huge molars contrast strongly with 

 the elongated canine tusks and diminutive molars of the 

 Dinocerata. The latter, moreover, have two very large depen- 

 dent processes on each ramus of the mandible ; the cervical 

 vertebrae flat; the femur without a third trochanter; and at 

 least an additional toe in each foot. 



Amojig the features which this group shares with the Prohos- 

 cidea may be mentioned : the superior extension of the condylar 

 "dge of the humerus ; the short thick toes ; and the late union 

 01 the epiphyses with the centra of the vertebrae. The last 

 character appears to belong especially to mammals of very large 

 ^^^^' and probably indicates late maturity, and great longevity, 

 ^\^ Brontotheridce nearly equaled the Elephant in size, but 

 tS^ m""^^ ^^^*^ shorter, the nose was probably flexible, as in 

 the Tapir, but there was evidently no true proboscis. 



