DIDELPHYID^E. 279 



forms 1 similar variations occur in the size of the true molars, and 

 pm. 4 may be either equal to or taller than either of the two pre- 

 ceding teeth. The talon of the last lower true molar may be either 

 bi- or tricuspid. The resemblance between lower jaws of existing 

 species from the Brazilian caves and those of certain Tertiary forms 

 is so close as apparently to leave little or no doubt as to their generic 

 identity. From the White- River group of North America certain 

 forms (fig. 42) have been referred to Peratherium, but are said to 



Fig. 42. 



d 



(^)fugax (Cope) ; from the White-Biver Miocene of Colorado, a, b, 

 inferior and lateral views of skull; o, d, superior and lateral views of 

 right mandibular ramus. f . (From the ' Amer. Nat.') 



differ from Didelphys by the non-inflection of the angle of the man- 

 dible 2 , although this feature is well marked in the European Tertiary 

 species. The figured American specimens do not, however, show 

 the angle ; but if the alleged absence of inflection be eventually 

 proved correct, it will indicate a very remarkable deviation from the 

 ordinary type. It should be observed that the number of incisors 

 in the European Tertiary forms is not known, but it was doubtless 

 the same as in existing species 3 . 



1 Peratherium was proposed on the grounds that the lower true molars 

 increased in size from the first to the last, which has a biscuspid talon, and that 

 pm . 4 was larger than either of the others ; while Amphiperatherium (see Filhol, 

 ' Ann. Sci. Geol.' vol. xii. art. 3, pp. 67-68) was made to include those forms 

 in which the lower true molars decreased in size, and pm. 4 was not the tallest 

 of the series. The latter forms are therefore identical in these respects with 

 species like D. lanigera. Another group is proposed by Filhol (see p. 54 of 

 memoir cited) for species intermediate in these characters between Peratherium 

 and Amphiperatherium. Graudry ('Les Enchainements, etc. Mamm. Tert.' 

 p. 11) regards Peratherium as undistinguishable from Didelphys. 



2 See Cope, Amer. Nat, vol. xviii. p. 687 (1881) ; it is there stated that the 

 inflection is also absent in the European forms. 



3 It is almost certain that the number of named Tertiary species will have to 

 be reduced, but the specimens in the Museum are not sufficiently numerous or 

 perfect to admit of this work being undertaken. In several instances a pro- 

 visional element enters into the following specific determinations. 



