44 MR. M. F. WOODWARD ON THE [Jan. 5, 



secondarily acquired character, developed comparatively late in the 

 evolution of the class. 



Detailed perusal of his writings shows these conclusions to be 

 drawn to a large extent from the study of the Marsupialia and 

 especially of Thylacinus ; the solitary tooth shed by that animal he 

 regards as the sole representative of the milk or first dentition of its 

 higher allies (Eutheria), believing the rest of its teeth to represent 

 the permanent or successional second dentition of the latter. At the 

 same time (7. p. 2) he has pointed out (and laid great stress on the 

 fact) that the milk-teeth of the Eutheria invariably show a more 

 primitive pattern and shape than those of the permanent or second 

 series which replace them. The latter are often highly specialized : 

 while the former often (as is especially the case with the Ungulates) 

 agree more or less closely with the permanent teeth of the extinct 

 ancestors of the order. 



He concludes that when one set of teeth only are present, as in 

 the Cetacea, it is invariably the permanent or 2nd one, the milk or 

 1st set being either not developed or suppressed. 



Arguing along the same lines, he considers that when a tooth 

 such as the 1st premolar in many diphyodont mammals is only present 

 in one dentition (even though in many cases it is very early lost), 

 it must belong to the 2nd or permanent series. 



Thomas (26, 27) has lately accepted Flower's views as to the 

 relation of the two dentitions, and has added largely to our know- 

 ledge of the dentition of Marsupials, Edentates, and Monotremes. 

 He shows conclusively that it is invariably the 4th premolar (not 

 the 3rd as Flower thought) which is replaced by a vertical successor 

 in the Marsupials, thus bringing the dentition of Marsupials and 

 Placentals into more complete harmony ; while among the Edentates 

 he has proved the existence of a milk-dentition in Orycteropus (26). 

 More recently Kiikenthal (18), in a preliminary account of some 

 researches on the development of the Cetacean teeth, has sought to 

 show that, exclusive of the Monotremes, there is no such thing as a 

 monophyodont mammal. In all Cetacea (the typical monophyodonts) 

 he finds that rudimentary successional teeth appear in connexion 

 either with the more fully developed functional ones of the toothed- 

 whales or with the functionless tooth-points of the Mysticeti. He 

 argues from this that these supposed typical Monophyodonts are 

 really modified Diphyodonts, and further that their functional or most 

 fully developed teeth belong to the 1st or milk-dentition, and not, 

 as Flower supposed, to the 2nd set. He also advances some reasons 

 for believing that the homodont condition may be arrived at by a 

 splitting up of the primitive complex teeth of an original heterodont 

 type. 



In the Marsupials \ from a careful examination of the developing 

 ^ Since the above was written Kiikenthal has published (Anat. Anz. 1891, 

 p. 658) the details of his work on Bidel-phys, giving figui'es of the rudimentary 

 successional teeth in connexion with the incisors, premolai-s, and even molars, 

 the last being thus shown in the Marsupials to belong to the 1st dentition. 

 These observations I can confirm so far as the incisors and molars of Didelphys 

 are concerned, but in Trichosurus I can find no trace of the teeth successional 

 to the molars. This may be owing to the embryo being too young. 



