556 SB. E. AMEGHiNo OX THE [May 2, 



several simple teeth. It is this latter theory which I have been 

 iipholding for the last 15 years. 



In a memoir published about three years ago \ I showed that 

 the tritubercular theory, contrary to what has been asserted, does 

 not agree with the facts furnished by either the embryology, 

 palaeontologv, or general morphology of the mammalian dentition. 

 I observed, moreover, that triconodonty and trituberciily, far from 

 being stages leading to the more complicated forms of teeth, are, 

 on the contrary, the result of the reduction of the latter. It was 

 not until after the publication of my paper that I learned that 

 Dr. Forsyth Major had expressed views similar to mine in the 

 Proceedings of this Society. 



As a complement to my preceding work, I shall now endeavour 

 to determine the most primitive type at present recognizable in 

 the crown of ihe lower plexodont molars of Mammalia. In a 

 subsequent paper I shall deal with the upper molars. 



Firstly a few words on the terms used by me. I recognize in 

 the dentition : — 



(1) The deciduous molars (milk-teeth) and the persistent 



molars (true molars), representing together the Jiri^t series, 

 which is the oldest from an embryological as well as from 

 a palseontological point of view. 



(2) The replacing molars (premolars), representing the second 



series, which is of more recent date and always remains 

 incomplete. 



I assign to the teeth behind the canines the progressive numbers 

 1 to 7, since they are perfectly homologous in the Placentals and 

 Marsupials, the only difference being that some teeth may belong 

 to the first series in certain genera {e. g. the fourth of Marsupials) 

 and to the second series in others (e.g. the fourth of Placentals). 



Each of the lower complicated molars exhibits two lobes, an 

 anterior and a posterior, and six cusps or denticles, three for each 

 lobp. According to the authors of the tritubercular theory, these 

 cusps have made their appearance gradually in successive geological 

 periods, and they assign to each cusp a different name. These 

 names have different suffixes in the molars of the two jaws ; 

 furthermore, there are different names for the same cusps in the 

 premolars, for the lobes according to their form, for the colon- 

 nettes (styles) and crests (lophs), <fec. ; constituting altogether such 

 a complicated terminology, that it remains absolutely unintelligible 

 for all who have not specially studied the argument, and discourages 

 many persons who wish to become initiated in the study of palaeont- 

 ology. Besides, these names correspond with conceptions which 

 are often uncertain and sometimes preconceived. 



I shall only make use of the old and vulgar names designating 

 the different parts according to their position. Every complete 

 plexodont molar has an anterior and a posterior lobe, ear-h of them 

 carrying three cusps. The three cusps of the anterior lobe are the 

 median-anterior, the antero-external, and the antero-internal ; the 



' F. Ameghino, " Siir rEvolntion des Dents clep Mnmmiferes," in Eol. Acad. 

 >'ac. de Cieiu-., t. xW. pp. 3«l-ol7 (1896). 



