1887.] NATURAL SCIENCES OF PHILADELPHIA. 287 



The chief molar types which characterize families are as 

 follows : 



A. Without opposition of cusps. (1) With three stout, erect 

 cusps in line. (2) Transitional. With three cusps not in line, two 

 being rotated inwards. (3) With three slender, divergent cusps 

 in line. B. With completely opposed cusps, sejoarated by a longitud- 

 inal valley. (4) C. With completely opp)Osed cusps connected by 

 transverse ridges. (5) Molars with a single styloid cusp on one side 

 of the crown connected by divergent ridges with a pair of cusps 

 on the other side. D. Without cusps. (6) The crowns columnar, with 

 a smooth wearing surface. 



By a careful study of the dentition and mandibular structure, we 



iind that these families unite in small groups which are in early 



stages of differentiation along certain lines of functional adaptation. 



These lines are not sharply defined, but by a comparison of the 



typical forms with the most nearly allied genera of known adaptation 



I find we may divide these sub-groups into carnivorous, omnivorous 



insectivorous and herbivorous series ; employing these terms in a broad 



sense as indicating an initial rather than an advanced specialization 



of structure. 



Carnivorous Sub-Group. 



There are many points of resemblance between the following three 

 families, although at first sight they differ widely. They mostly 

 embrace the largest genera and the teeth are generally adapted to a 

 carnivorous diet, especially in the first and second families; in the 

 third there is an apparent divergence towards another type of dentition 

 and function. The molars have a strong internal cingulum. The 

 premolars have basal cusps. The condyle is low and the coronoid 

 broad. 



A. (1) Triconodontidic} The typical genus of this family is 

 Triconodon (syn. Triacanthodon, Owen; Priacodon, Marsh.) The 

 dentition is now fully known. There is no diastema. The premolars 

 have strong basal cu.sps. The canines and the incisors are erect and 

 powerful. The mandible is strong and the condyle is below the 

 molar level. These characters and the likeness to Thylacinus show 

 that this was a carnivorous animal. An older and much less 

 specialized but probably allied form was Amphilestes. Amphitylus 

 appears to be related by the character of its molars, but separated 

 by its mandibular characters, as the condyle is high and pedunculate. 



1. Proposed by Professor Marsh. 



