1888.] NATURAL SCIENCES OF PHILADELPHIA. 301 



of Leptocladus dnbiiis and Sjxdacotherium minns W\i\\ Percmms, and 

 determination of the mandibular dentition of the latter o-enus. The 

 molars are tritubercular. 5, The discovery also of apparently tritu- 

 bercular molars in Amphitherium and probable determination of the 

 premolar- molar formula, (confirming Owen's views). 6, Confirming 

 Lydekker's suggestion of the probable union of Peralestes with Spala- 

 cotherium, and of Peraspalax with Amhlotherium. 7, The probable 

 union of Peraspalax, Amhlotherium, Achyrodoii, Phascolestes, Stylodon, 

 and Kurtodon, into two or three genera with a substantially similar 

 molar structure. 8, Th» correction of the writer's former views as 

 to the family separation of the Peralestidce and probably of the 

 Kurtodontidce. 



The general result of the renewed and more extended study of these 

 mammals has thus been, first to reduce the number of genera and 

 eliminate tw^o of the families proisosed in the Memoir ; second, by the 

 discovery of the molar structure of Amjohitherium and Peramns, to 

 substantially reduce the number of molar types among the English 

 genera to two, viz. : the triconodont in Amj)hilestes,Phascolotherium, 

 Tricoiiodon and probably AmiMtijlus, and the tritubercular in all the 

 remaining genera. 



This latter result is of great interest in its bearing upon the theory 

 that the molar teeth of all the mammalia have either passed through 

 the tritubercular stage or have been arrested at one of the steps in. 

 tooth development leading to this stage. 



