22 EVOLUTION OF MAMMALIAN MOLAR TEETH 



I. S'libfamily Triconoclontince Osborn. 



Upper Jurassic.* Anterior and posterior cusps of molars progressively 

 increasing in size, finally equalling the middle cusps. All three cusps in 

 line. Postcanine teeth 7. Angle not seen in outer, side view. Canines 

 erect and piercing. Mandible stout. 



1. Triconodon Upper Jurassic,* England (Middle Purbeck Beds). 

 Figs. 11, 11 a. 



2. Priacodon (probably a synonym of Triconodon), Upper Jurassic 

 (Como Beds), Wyoming. Fig. 8. 



3. Triacanthodon, Upper Jurassic * (Middle Purbeck Beds), England. 

 Fig. 7. 



c. Subfamily Phascolotheriinm Osborn. 



Lower and Upper Jurassic. Anterior and posterior cusps of lower 

 molars progressively shifting to inner slopes of central cusps. Postcanine 

 teeth 7. Premolars and molars alike (or premolars greatly reduced in 

 number). Angle not seen in outer side view. 



1. Phascolotherium, Lower Jurassic (Stonesfield Slate), England. 

 Figs. 4 (No. 3), 6. 



2. Tinodon, Upper Jurassic * (Como Beds), Wyoming. Fig. 10. 



INCERT^E SEDIS. 

 d. Subfamily Spcdacotheriince Osborn. 



Upper Jurassic. Molars with large central cusp and two smaller 

 usps somewhat internal to it. Premolars and molars unlike (or pre- 

 molars not greatly reduced in number). Angle not seen in outer side 

 view. Postcanine teeth 7-10. Mandible slender. 



Spalacotherium, Upper Jurassic """ (Middle Purbeck Beds), England. 

 Fig. 11. 



Menacodon, Upper Jurassic (Como Beds), Wyoming. Fig. 9. 



Peralestes, Upper Jurassic * (Middle Purbeck Beds), England. 



B. Infra-class Placentalia. 



ORDER : PANTOTHERIA MARSH (TRITUBERCULATA OSBORN). 



Probably insectivorous pro-placentals, angle of mandible not inflected, 

 lower molars primitively of the tuberculo-sectorial type, with a central 

 external and three internal cusps, pa d , me d , hy d , the latter probably 

 derived from the internal cingulum. Upper molars subtriangular, the 



* [Throughout this book the Purbeck Beds of England and the Como or Atlantosaurus 

 Beds of Wyoming are called Upper Jurassic, but English and Continental geologists now 

 regard them as Lower Cretaceous in age. ED.] 



