1899.] PLEXODONT MOLARS OF MAMMAX8. 565 



much accentuated, the six primitive cusps, as also a trace of the 

 cingulam, c ; the chief difference from Proteodidelphys being seen 

 in cusp mn, which has moved to the inner side. Fig. 13 repre- 

 sents the same molar of the Eocene genus Piiheculus, a Monkey of 

 the family Homunculidae. This tooth is more square and has lost the 

 indentation on the internal side of each lobe ; the cusps are more 

 in the shape of mounds, while the median anterior cusp is veiy 

 small, forming part of an anterior crest, from which it is scarcely 

 distinct. In Hornunculus, of the Upper Eocene, the same tooth 

 (tig. 14) shows the median-anterior cusp ma to have become effaced 



Hornunculus pataqonicus : fifth right lower molar, superior (a) and external (6) 

 aspect, four times nat. size. — Upper Eocene ; Patagonia. 



by fusion with the anterior crest, whilst the tubercular or buno- 

 dont form is more pronounced. In recent Monkeys and in Man 

 the transverse anterior crest, the last vestige of cusp ma, has also 

 disappeared, there remaining only the four cusps ae, ai, pe, pi^ 

 which are in the form of mounds or tubercles almost equal in size 

 and imparting to the crown the perfect omnivorous aspect. The 

 cusp mp often remains visible, generally placed between the two 

 posterior cusps pe and pi, but always of minute size. 



At different times I have supported the contention that the 

 complicated molars of Mammalia have retained the same form 

 from one end of the series to the other, with no other change than 

 that of the relative size of their different parts. On this hypothesis, 

 the simplification of the deciduous molars and of the premolars 

 must be considered as a secondarily acquired character, due to the 

 want of space for the complete development of these teeth — a 

 simplification which must have been acquired progressively from 

 before backwards. 



I have insisted on the fact that the deciduous molars, although 

 remaining in function for a short time, are almost always more 

 complicated than those which replace them. This is in agreement 

 with the theory of fusion and primitive complication, since the 

 deciduous teeth are the older dentition of the two ; but it is in 

 contradiction to the theory of gradual complication. I have 

 also drawn attention to the fact, almost universal in Placentals 



Pkoc. Zool. Soc— 1899, No. XXXVII. 37 



