EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHEKES 



261 



there is no other Greek word descriptive of the head 

 as a whole. We can not use "dolichocranial," because 

 our indices measure the whole skull, not the calrarium 

 alone. 



The standard zygomatic-cephalic indices adopted 

 in this monograph are as follows: 



Dolichocephalic 39-60 



Mesaticephalic 60. 1-70 



Brachycephalic 70. 1-91 + 



It is true that in the top view of Brontoiherium 

 platyceras the skull top itself is long, but the indices 

 show that the head as a whole is extremely brachy- 

 cephalic. In uncrushed skulls of Bro7itotherium gigas 

 the index is 82-S7, which is doubtfully exceeded only 

 by B. rohustus and Diploclonus amplus. 



CONTRAST IN FEATURES OF BRACHYCEPHALIC AND 

 DOLICHOCEPHALIC SKULLS AND TEETH 



The later brontotheres are excessively brachyopic, 

 not only in measurements but in all the characters of 

 the teeth, including molars and premolars. This 

 brachyopy, no doubt, supervened upon an earlier 

 stage in which the middle portion of the cranium was 

 elongate, and it is the elongation of the middle part 

 of the cranium that gives a dolichocephalic tendency. 

 Far from being elongate as viewed from below, the 

 palatal and basicranial regions of brontotheres are 

 excessively wide and short, as well as the zygomata 

 and the face, and thus contrast very strongly with the 

 narrow face and unexpanded zygomata of Menodus. 

 Hence there is no available substitute for the term 

 brachycephaly for the brontotheres. 



Contrasts in features oj hracJiycepJialic and dolichocepTialic teeth and slculls 



Brachycephalic type 



DolichocephaUc type 



Teeth: 



Grinding series 



Diastema between cutting and grinding 



Anterior premolars, p \ 



Intermediate tubercles or conules of 

 molars. 



Opposite cutting and grinding series 



Incisor series 



Canine teeth 



Grinding teeth, or molars 



Cingulum between molar teeth 



Skull: 



Entire skull 



Most of the constituent bones 



Palate 



Nasals 



Abbreviated. 

 Closed 



Suppressed, or one fang suppressed. 

 Persistent 



Bridge over infraorbital foramen. 



Infraorbital foramen 



Lacrimal bone 



Lacrimal foramen 



Zygomatic arches 



Areas of insertion of masseteric and 



temporal muscles. 

 Mastoid portion of periotic 



Exoccipital, postglenoid and post-tympanic 



processes. 

 Postglenoid and post-tympanic processes. . 



Tympanic bulla 



Foramen ovale and foramen lacerum 

 medius. 



Alisphenoid canal 



Presphenoid 



Vomer 



Premaxillary symphysis 



Frontonasal horns 



Converging or arched. . 



Placed transversely 



Rounded or broadened. 

 Shortened and widened. 

 Suppressed 



Shortened and broadened . 

 do 



Broadened and flattened.. 

 Shortened and spreading. 



Narrowed 



Not seen on side of face 



Crowded toward orbit 



Crowded into orbit 



Broadened, especially in the buccal 

 plates; in section broad rather than 

 deep. 



Increased in thickness 



Exposure abbreviated or covered. 

 Broadened 



Jaw: 



Ramus of jaw. 



Area of insertion of temporal muscle. 



Coronoid process 



Mandibular symphysis 



Approximated, especially below, in- 

 closing the external auditory meatus 

 inferiorly. 



Thrust inward 



Approximated 



Abbreviated 



do 



Thrust backward 



Abbreviated and massive 



Transversely expanded 



Shortened, thickened, deepened 



Reduced 



do 



Abbreviated and massive 



Elongated. 

 Open. 



Persistent and spaced. 

 Reduced or aborted. 



Parallel or elongated. 

 Convergent anteriorly. 

 Elongated or compressed. 

 Lengthened and narrowed. 

 Persistent. 



Lengthened and narrowed. 



Do. 

 Narrowed and transversely arched. 

 Elongated with curved and straight 



borders. 

 Broadened. 



Conspicuous on side of face. 

 Exposed on side of face. 

 Seen on edge of orbit. 

 Elongated and vertically deepened; in 



section deep rather than broad. 



Elongated horizontally. 



Expanded and exposed.- 

 Deepened and narrowed. 



External auditory meatus not closed 

 inferiorly. 



Exposed laterally. 



Separated by a bridge o i bone. 



Elongated. 



Do. 

 Not thrust backward. 

 Elongated. 

 Less expanded transversely. 



Elongated with straight lower borders 



and backward produced angle. 

 Balance maintained. 

 Lengthened anteroposteriorly. 

 Elongated. 



