298 I'rof. 0. C. Marsh — American Jurassic Mammals. 



order, whicli the writer has termed the AllotJieria,^ can be distin- 

 guished from the previous group by the following characters : — 



(1.) Teeth much below the normal number. 



(2.) Canine teeth wanting. 



(3.) Premolar and molar teeth specialized. 



(4.) Mylohyoid groove wanting. 



(5.) Angle of lower jaw distinctly inflected. 



These characters alone do not separate the Plagiaulacidce and 

 Microlestidce from some of the Marsupials, and the facts now known 

 seem to prove that they belong to that group, where they represent, 

 at least, a well-marked sub-order. 



Of the two families of Triassic Mammals now known, the Dromo- 

 theridcB may be placed in the order Pantotheria, and the Microlestidce 

 in the Allotheria. According to present evidence, the former were 

 probably placental, and the latter non-placental, and marsupial. 



The modern Placental mammals were evidently not derived from 

 Marsupials, as is generally supposed. Each group has apparently 

 come down to the present time, by separate lines, from primitive 

 oviparous, forms, of which the living Monotremes may be the more 

 direct but specialized representatives. Among the diversified mem- 

 bers of Placental mammals, the Insectivores are probably the nearest 

 to the early type, and hence they show many features seen in the 

 Jurassic and Triassic mammals of the order Pantotheria. 



Among the various existing Marsupials, the Eat-Kangaroos 

 (^Sypsipi-ymnidce) appear to be nearest to the oldest known forms 

 represented in the order Allotheria, but future discoveries may, at 

 any time, bring to light new Mesozoic mammals allied to other 

 Marsupials, 



So far as at present known, the two great groups of Placental and 

 Non-placental mammals appear to be distinct in the oldest known 

 form, and this makes it clear that, for the primitive generalized 

 forms (JBypotheria) , from which both were derived, we must look 

 back to the Palgeozoic. 



EXPLANATION OF PLATES VIII. and IX. 

 PLATE VIII. 



Fig. 1. Left lower jaw of Stylacodon gracilis, Marsh ; outer view. 



2. Left lower jaw of Bryolestes prisons, Marsh ; inner view. 



3. Left lower jaw of Bryolestes vorax. Marsh ; outer view. 



4. The same jaw; inner view. 



5. Left lower jaw of Laodon venustiis. Marsh ; inner view. 



6. Left lower jaw of Asthencdon seg)iis. Marsh ; anterior part, outer view. 



7. Eight lower jaw of same species ; outer view. 

 a, canine ; b, condyle ; c, coronoid process ; d, angle ; g, mylohyoid groove ; 



s, symphyseal surface. 

 AU the Figures are three times natural size, except Figure 5, which is four times 

 natural size. 



PLATE IX. 



Fig. 1. Eight lower jaw of Tinodon bellus, Marsh ; inner view. 

 ,, 2. Eight lower jaw of Doco^ow s^na<2«s, Marsh; inner view. 

 ,, 3. Eight lower jaw of Biphoeynodon victor, Marsh ; outer view. 



1 American Journal of Science, vol. xx. p. 239, 1880. 



