PALEONTOLOGY. 301 



orons sub frroup, D. (6) Athrodontida*, of which the j?enus Athrodon 

 (geri. nov.) is based upon the maxilla of iStylodon pmillus (Owen). 



The Multitubercidafa are regarded as a sub-order of the Marsupialia^ 

 while the second suborder presents no characters associating them with 

 known Marsupialia. 



Osborne (215) gives additional observations upon the structure and 

 classification of Mesozoic mammals, and offers the following summary, 

 pp. 300-301, of the principal features of this contribution : 



" The principal features of the present contribution are the follow- 

 ing : ( 1 ) Additional characters of Amphilestes and the probable determi- 

 nation of the premolar-molar formula. (2) Additional characters of 

 FhascolotJierium, suggesting a division between molars and premolars. 

 (3) A review of the Amphitylus dentition. (4) The union of Leptocladus 

 dnbius and SpaJacotherium minus with Peramns, and determination of 

 the mandibular dentition of the latter genus. The molars are trituber 

 cular. (5) The discovery also of apparently tritubercular molars in 

 Amphithcrium and probable determination of the premolar-molar for- 

 mula (confirming Owen's views) ; (G) confirming Lydekker's suggestion 

 of the probable union of Peralestes with Spalacotheriiim, and of Pera- 

 spalax with Amblotherium ; (7) the probable union of Peraspalax, Am- 

 blotherium, Achyrodmi, Phascolestes, Stylodon, and Curtodon into two or 

 three genera with a substantially similar molar structure; (8) the cor- 

 rection of the writer's former views as to the family separation of the 

 Perolesfidw and probably of the Curtodontidw. 



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

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

 inate two of the families proposed in the memoir; second, by the dis- 

 covery of the molar structure of Amphitlierhim and Peramns, to substan- 

 tially reduce the number of molar types among the English genera to 

 two, viz, the triconodont in Amphilestes, Phascolotherium, Triconodon, and 

 probably AmphityUis, 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.'' 



Osborne (213) gives a short abstract of the full paper (214), which was 

 published in the December number of the American Naturalist, 1888. 

 The author proposes the following nomenclature for the cusps of the 

 upper molars: 



Protocone, for the antero-internal cusp. 



Hypocone, for the posterointernal cusp, or sixth cusp. 



Paracone, for the antero-external cusp. 



Metacone, for the posteroexternal cusp. 



Protocotiule, for the anterior intermedial cusp. 



Metaconule, for the posterior-intermedial cusp. 



