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Show 568 SR. F. AMEGHINO ON THE [May 2, Microbiotheridae, but no traces of it are to be seen in recent Didelphyidae, 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 Homunculus 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 Cebidae, 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 posterointernal 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 molars 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 another, 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 Notopithecidoe, 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 Isotemnidae, tbe Astrapo-theridae, &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 1 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 m y notice. I have just received Prof. Osborn's memoir on the " Evolution of Amblypoda, Part I.," in Bull. Amer. Mus. of Nat Hist xi 18981 and on page 172 I find the figure of the mandible of Pantolambda' cavinctus. 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 Proteodidelphys, Protwotherium Homunculus, &c. r ' |