CHAPTER VI 1 1. 

 EVOLUTION OF THE PREMOLARS. 



IN the early geological periods the preinolar teeth of mammals were 

 very simple in structure, as shown in the following description and 

 figures by Osborn ] of the premolars of the Triassic and Jurassic 

 mammalia : 



1. PREMOLAES IN PRIMITIVE MAMMALS. 



"The premolars of DromatJierium [Fig. 195^4] are very unique. They 

 are tall and styloid and single fanged ; the last premolar has a vertical 

 groove upon the posterior face. In Microconodon [Fig. 195#], which 



A B 15 4 3 7 // /2 A5 



FIG. 195. The premolar tooth forms of the mammals of the Mesozoic order Pantotheria 

 or Tritnberculata. The premolar represented is invariably the most posterior of the series ; 

 the anterior face is to the left. A. Dfunmtlt: i-ium, B. Microconodon. 15. Ki'i'tixlii,!. 4. Ti'ico- 

 nodon. 9. Peraspalax. 1. SpalcLcotheriwrn. 11. Phascolestes. V2. Dryolestes. > 3. Achyrodon. Nos. 

 A, 4, 9, 11, 12 are seen upon the inner surface, the remainder upon the outer surface. 



belongs to a somewhat more recent type, the premolars have a faint 

 posterior heel, and the last shows the trace of a double fang. In all 

 the Jurassic genera the premolars, where fully functional, are bifanged, 

 and possess a convex anterior face and concave posterior slope terminating 

 frequently in a heel. As in the molars, the cingulum plays an important 

 part in connection with the basal cusps. It is present upon the internal 

 face of the premolars of all the Jurassic genera except Kurtodon 

 [Fig. 195 15], and is observed upon the outer surface in Diplocynodon 

 [Fig. '20]. It thus in many cases enables us to draw the line 

 between premolars and molars, as in both the Peralestid;u J'< fn/rstes, 

 Per<(xj>r//ii.<' [Figs. 12, 22, 195 9], and in the genera of the Insectivorous 

 Sub-group the inner faces of the molars are smooth. 



1 0sborn, H. F., "The Structure and Classification of the Mesozoic Mammalia," 

 Acad. Nat. ,SV., Philadelphia, Vol. IX., No. '2, July, 1888, pp. iZo-^-'G and pp. _':;'.i--_ ) 4it. 



N 



