ia 
aes 
i: 
1884.] The Tertiary Marsupialia, 693 
of one of the most interesting of living forms. œ On the other 
hand, the Polymastodontidz may well have. derived their origin 
from the Tritylodontide, which were also of ‘herbivorous or 
granivorous habits, The family of the Plagiaulacidz is one of 
the most peculiar among those of the. Mammalia, whether we 
consider its structure or its relations to geological time. Com- 
_ mencing in the Jurassic period, it persisted through the Creta- 
= Cous to the Eocene. It then disappeared from view to remind 
= usonce more of its existence by its probable descendant, the ex- 
~ traordinary pouched lion of the Pliocene period of Australia, 
Thylacoleo carnifex Owen (Fig. 9). The family exhibits the usual 
= Successional relation of its component genera. In this respect 
__ itrepeats what I have already pointed out as a law of succession 
in placental Mammalia} a reduction in the number of premolar 
_ teeth. The following table exhibits these relations: 
| I. Tubercles of superior molar crescentic. 
Fourth premolar serrate, not ridged : Meniscoéssus 
Il. Tubercles of molars subconic. 
i 
l a. Four compressed premolars below. 
i Premolars serrate, not ridg ict eee Ctenacodon 
Premolars ridged and serrate . i AR Plioprion$ 
aa. Three compressed premolars. 
Premolars ridged. ..... ; Plagiaulax. 
aaa, Two premolars. y p 
a. ee a Prilodus, 
aaga. One premolar.* 
Fourth premolar ridged ...... SE ane pts Pa Ae es Neoplagiaulax. 
Fourth Premolar smooth., Pees eet eee eseees Liotomus, x 
= these seven genera but nine species are thus far known. 
lodus and Ctenacodon have two species each; and each of the 
gh but one. Ctenacodon, Plagiaulax and Plioprion are Juras- 
; Meniscoéssus is Cretaceoùs, and the remaining three genera 
owned Tertiary. The American genera are Ctenacodon 
Pe (Fig. 7 £g), Meniscoéssus Cope, and Ptilodus Cope. The 
„amed is the most generalized of the family. The Menisco- 
ESSUS con 
quisius Cope, has the distinction of being the only known 
of the Cretaceous period. 
Im- ra K 
Eaa family iS the equivalent of Marsh’s “ order” Allotheria. 
: Rulletin U, s Geol. i 
“ N 
Gen, ber unknown in Liotomus 
You. 
