H. F. shorn — Mammalia in North America. 391 



reverence for the Marsupial stem theory. JSTow, it is very 

 significant that the average dentition of these old but highly 

 diverse forms, namely, Multituberculates, 3 ?, 4, 6, Tricono- 

 donts, 4, 1, 4, 7, Trituberculates, 4, 1, 4-5, 8, is also the denti- 

 tion to which the existing mammals apparently revert. 



Thus, the tendency of late research is to show that all stem 

 mammals were related in their double succession, in their 

 dental formula, and in their primitive molar form. These 

 features point, not to a succession, but to a unity of ancestry 

 of the Monotremes, Marsupials, and Placentals. 



Divergence of the three Groups. 



The discovery of the complete double series seems to have 

 removed the last straw from the theory of the Marsupial 

 ancestry of the placentals, for the peculiar mode of suppres- 

 sion of the second series in the Marsupials has been constant 

 since the Purbeck ; this difficulty is added to the structure of 

 the jaw, the epipubic bones, the profoundly different mode of 

 foetal nutrition. None the less, any conclusion we can draw 

 now as to the primary relations of the three great groups is 

 more or less of a " Schwindelbau," and I put together the 

 results of these later discoveries with a full realization of the 

 temporary character of present conclusions. 



The Permian Sauro-Mammalia (Baur) with a multiple suc- 

 cession of simple conical teeth divided into : A, Theromorpha, 

 which lost the succession and in some lines acquired a hetero- 

 dont dentition and triconid single-fanged molars ; B, Pro- 

 mammalia. 



The hypothetical lower Triassic Promammalia retained a 

 double succession of the teeth; they became heterodont, with 

 incipient triconid double- fanged molars ; dental formula ap- 

 proximating 4, 1, 4-5, 8. They gave rise to three groups; 

 I. The Prototheria which passed rapidly through the trituber- 

 cular into the multitubercular molars in the line of Multituber- 

 culates, and more slowly into trituberculy and its later stages 

 in the line of Monotremes. II. The Metatheria or Marsupials 

 tended to suppress the second series of teeth, except those 

 intercalated with the first ; by this and by reduction the 

 formula became 5, 1, 3, 4-6 ; the molars passed slowly through 

 the triconodont into the typical tritubercular type. III. The 

 Eutheria or Placentals divided early into a number of branches, 

 in which there was heterodontism, but no uniform modification 

 of succession, namely : A, forms suppressing the second series 

 in the molar region only, and acquiring a typical Eutherian 

 dentition, 3, 1, 4, 3-4. 1. The Insectivores tended to partly 



Am. Jour. Scl— Third Series, Vol. XLVI, No. 275.— Not., 1893. 

 27 



