6o AMERICAN MESOZOIC MAMMALIA 



Mi and M5 are the largest and highest of the molars, the series decreasing gradu- 

 ally both ways from these. All of the molars are closely similar in structure, and are of 

 very primitive tuberculosectorial type, with distinct trigonid and talonid. The trigo- 

 nid consists of the three primitive cusps only. The pr"* is high, simple, and vertical, 

 approximately conical but sending toward the pa'' and toward the me"^ distinct shear- 

 ing crests which begin at its apex as angulations in the otherwise circular horizontal 

 section. The me'' is directly internal to the pr'', apparently a trivial feature but prob- 

 ably one of much functional and evolutionary significance. The me'' is simple, colum- 

 nar, bluntly pointed, and is connected with the pr"' by a shearing transverse crest as 

 already noted. This crest bears a deep V-shaped notch between the pr^ and me''. The 

 pa'' springs forward and upward at an angle of about 45°. It is united to the pr'' by an 

 oblique, notched, transverse crest, and its base reaches that of the me'' so that the trigo- 

 nid is basined. The pa'' is much lower than either of the other cusps. On Mi the trigo- 

 nid is about as long as broad, but on the succeeding molars it becomes progressively 

 more compressed anteroposteriorly, reaching a maximum on Me. Wear on the trigonid 

 is of two types : The oblique anterior border of the tooth, involving the pa'', pr'', and the 

 crest between them is worn by shear, developing a facet at an angle of about 75° to the 

 horizontal, and a second facet of wear at about the same angle, but dipping posteriorly, 

 involves the posterior side of the pr'', the me'', and the crest between them. With more 

 advanced wear these two surfaces become confluent and more horizontal, truncating 

 the whole trigonid. 



The talonid is narrow, but is present on all molars. First appearing very low on 

 the crown beneath the pr''-me'' notch, it runs inward and upward, widening as it goes, 

 and culminates in a low cusp directly posterior to the me''. It is not basined. 



Each molar has two roots. One is very small and is confined to the posterointernal 

 part of the tooth, below the talonid cusp. The other is large and supports the antero- 

 internal and whole external part of the tooth. In internal view two apparently equal 

 roots are thus seen, while in external view but one appears. 



Mandible 



The horizontal ramus is long and slender, the alveolar border nearly straight in 

 the molar region, the lower border gently convex. The symphysial surface, where 

 union was ligamentous or cartilaginous only, is long, ending posteriorly beneath P3. 

 The internal groove is well marked. It begins just anterior to the dental foramen and 

 runs forward to the symphysis, gradually converging toward but not reaching the 

 lower border. The coronoid process is incompletely known, but was apparently long 

 and high. The masseteric fossa is broad and shallow and not sharply delimited. The 

 dental foramen is at the extreme anterior end of the equally shallow pterygoid fossa. 

 The angle is sharply differentiated, but is small, styloid, and projects straight back- 

 ward, continuing the curve of the lower border. There is apparently but one mental 

 foramen, beneath the posterior root of P2 and anterior root of P3. 



