PALEONTOLOGY. 301 



orou8 sub jjroup, D. (0) Athrodontida^, of wliich tho gomis Atlirodon 

 (geu. nov.) is based iii)oi» the maxilla of iStylodon pusUhis (Owen). 



The Mnltitiiherculat((, are reg^arded as a sub-order of tlie Mar.siqnalia, 

 while the secoud sub-order preseuts no characters associating them with 

 known Marsupialia. 



Osborne (215) gives additional observations upon the structure and 

 classification of Mesozoic mammals, and offers tlie following summary, 

 pj). 300-301, of the principal ieatures of this contiibution: 



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

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

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

 Fkascolotheriuni,si\gg(iiit\n^ a division between molars and premolars. 

 (3) A review of the Amphitiilus dentition. (4) The union of •Lepfocladus 

 dnhiu.s iind Sjxdaeothcyium minus with PeramuH^ and determination of 

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

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

 Amphiihcrlnm and probable determination of the i)remolar-molar for- 

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

 of the probable union of Peralcstes with Spalacotherium^ and of Pera- 

 spnJax with Amblotherium ; (7) the probable union of Pcraapalax, Am- 

 hlotherium, Aehyrodon, Phascolestes, Stylodon, and Curfodon 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 

 Peralestidw and probably of the GurtodonUdiv. 



" 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 Amphithcrium and Peramus, to substan- 

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

 two, viz, the trieonodont in Amphilestcs, Phasvolotherium, Triconodo7i, and 

 probably Amphitylus, 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 develoi)ment leading to this stage.'' 



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

 published in the Deceml)er number of the American Naturalist, 1888. 

 The author proposes the following nomenclature lor the cusps of the 

 upper molars: 



Protocone, for the antero-internal cusp. 



Hypoconc, for the postero-internal cusp, or sixth cusp. 



Paracone, for the antero-external cusp. 



Mefacoiie, foi- the i)ostero-external cusp. 



Protoconule, for the anterior-intermedial cusp. 



Metacomdc, for the posteriorintermedial cusj). 



