568 SE. p. AMBGHINO ON THE [MaV 2, 



Microbiotheridae, but no traces • of it are to be seen in recent 

 Didelphyidse, although the molars have again assumed their 

 original longitudinal disposition. 



These observations can be confirmed by the examination of all 

 the old groups of Mammalia. Not wishing to pass all of them in 

 review, I limit myself to the Primates, the great antiquity of 

 which had not been guessed before their discovery. 



The genus Homuncuhis of the Patagonian Eocene — a true Monkey 

 with rather specialized characters — is particularly interesting. Its 

 lower premolars, seen from the outer side, exhibit a single convex 

 lobe as in the Cebidse, and totally different from the persistent (true) 

 molars, which bear two well-developed lobes. Nevertheless, on 

 examining these same premolars from the inner side or from above, 

 they present a completely different appearance. These teeth are 

 seen to be inserted obliquely or almost transversely, so that they 

 show on the outer side only the enlarged anterior lobe with the 

 three well-developed primitive cusps ; whereas the posterior lobe 

 has moved inside and is partly atrophied, showing only the postero- 

 internal cusp pi, and the postero-external pe, which has moved 

 inside and with which the median posterior cusp has become 

 fused. 



In the line of the Primates the anterior molai's have therefore 

 also possessed the same form as the posterior ones, their secondary 

 and recent simplification being due to the want of the space 

 necessary for their development. The premolars, in consequence 

 of being pressed together, have assumed an oblique position, partly 

 overlapping one auother, and producing the atrophy of the posterior 

 lobe, which is no longer visible in the same teeth of more recent 

 Monkeys and of Man. In the Primates this atrophy began during 

 the Cretaceous, since it is already to be seen in the Notopithecidae, 

 all the members of which exhibit the same oblique insertion of the 

 anterior molars. I have also found it in several lines of Ungulates, 

 especially in the Protypotheridae, the Isotemnidse, the Astrapo- 

 theridse, &c. I draw the conclusion that the plexodont molars of 

 Mammals, the anterior as well as the posterior, had originally the 

 same degree of complication, and that the simplification of the 

 anterior molars, observable in numerous Mammals of the latest 

 Cretaceous and of the beginning of the Tertiary, is a secondarily 

 acquired character. This simplification was the outcome of a 

 concentration of the dental series, by want of the necessary space 

 for their development \ 



The diminution of the space assigned to the development of the 



■^ To those desirous of becoming acquainted with a similar instance in a 

 mammal of the Northern Hemisphere, I will point out one which at this moment 

 comes under my notice. I have just received Prof. Osboru's memoir on the 

 " Evolution of Amblypoda, Part I.," in Bull. Amer. Mus. of Nat. Hist. xi. 

 1898 ; and on page 172 I find the figure of the mandible of Paniolambda 

 caviHctus. A glance at this figure shows that in this ancient genus the 

 premolars are inserted obliquely, the posterior lobe being turned inwards 

 and atrophied in the same manner as in ProteocUdeVphi/s, Protypotherinm, 

 Homuncidus, &c. 



