448 SCIENCE PROGRESS. 



of the trias of North America, which may therefore be as 

 old as the multituberculate Tritylodon, the molar of which 

 Osborn hopes soon to prove to be itself a specialised 

 derivative of the tritubercular type. 



On the other hand, Forsyth Major, who has for years 

 paid special attention to this subject (42), is strongly of 

 opinion that the primitive mammalian molar was polybunous 

 in character ; and if this can be proved to be the case the 

 coalescence theory will naturally remain the simplest and 

 most natural explanation of the origin of this type of tooth 

 from the reptilian cone. 



Reconsideration of the above-cited facts leads us to the 

 belief (i.) that the living mammalia show traces of from 

 three to four distinct generations of teeth, and consequently 

 (ii.) that they are potentially polyphyodont, (iii.) that the first 

 set is vestigial and not functional in any living mammal, 

 (iv.) that the second, which is so important in the lower mam- 

 mals, is more or less replaced by the third in the higher 

 forms, and (v.) that this third dentition remains dormant in 

 the marsupials and cetacea. 



The point requiring immediate and renewed investiga- 

 tion is clearly the relation and mode of origin of the molar 

 teeth, but I do not think we shall be able to make much 

 advance towards the further solution of the general problem 

 by embryological so fully as by palajontological research. 

 If we are to regard it as proved that the triconodont tooth 

 of the reptiles is formed by the fusion of three distinct 

 teeth, we may safely apply the coalescence theory to the 

 mammalian molar, but it is not likely that the latter 

 would retain any very distinct indications of its originally 

 distinct elements, especially if, as seems probable, the mam- 

 malia inherited their triconodont tooth, ready formed from 

 their reptilian ancestor, in which case the early stages in the 

 development would probably by now have been suppressed 

 and the whole genesis abbreviated. 



In the Ritter lecture, delivered at Jena in 1892, by Kli- 

 kenthal (29), some very interesting views were put for- 

 ward as to the origin of the mammalia, with special refer- 

 ence to the dentition, which he would derive from some 



