ox the structure and classification 
Whifli miched too great a degree of specialization and dental reductioiD at the close 
of the Cr<-taceous to survive or leave descendants in the recent period. Whether they 
are to be considered as a branch of the monotreme or of the marsupial stock is an 
juisettled (piestion. 
B. Second Group. 
As we have seen upon pages 212 and 223, the mammals of this group are so dis- 
tinct from the Multituberculata that their zoological position must be considered sei> 
arately, and, unlike this order, they conform so little to a common type that when the 
approximate systematic position of one genus or family has been determined, it by no 
mean.s settles the que.stion in regard to the remainder. Do they belong to a distinct 
order? .\re they exclu.sively Marsupials or Insectivores, or do they stand in ancestral 
lines leading to each of these orders ? These are the three forms of the problem, which 
are conilitioned by the wider question whether the Placentalia have ever passed 
through the marsupial stage, with a peculiar yolk-sac placenta^ and restricted milk 
<lpntition. 
In the conclusion of his memoir (page 113), Professor Owen expressed his views 
ns follows: “Among these initial forms of Marsiqnalia we may see in Amithitherium 
the prototyi)eof Myrm€(x>bim ; Slylodoii has its analogue in ClirysocMoi'k ; Peralestes 
ha.s culminated in Surcoji/nlm; Triconodon in Thylacinus; Plagiaidax is to Thrjla- 
.«/« what tlic wea.sel is to the lion.” On page 111, he suggested that we found here 
also among those genera, in which marsupial characters were less clear, early forms 
of m«Klcm Insectivorn, but gave no specific grounds for this view. The prevail- 
ing opinion among paheontologists that the jurassic mammalia are all to be classed 
with the Marsupials, has been recently adopted by Lydekker.^ In reference to the 
lK>lyprot(^ont genera l.e writes : “The majority of which appear so nearly related to 
existing Marsupials that it has been a question whether some of them should not be 
...rludcl m ,l,a families.” In proposing the order PanWierin, Marsh in 
ISSO mul „p.,n 1887,* expre,sscd the diver* opinion that the mesozoic mammalia 
rannot 1» ^fslnctor, y pined in any of the recent orders: ” With a few exceptions 
Inv IZr host preservd are manifestly low generalized forms, without 
dl^ .T r P“”‘ ">”<= 
transfer them ti***^V*H^^^*' evidence based on specimens alone would 
transfer them to the latter group if they are to be retained in any modern order.” 
lion. 
v„,x..o Placenta in Didelphys.” 
• Cmt. Fom. M.njin.,'PaH V, p. ix. 
Mm. Jour. .Sp., Vol. XX, p. 239, 1880 
•■im Jour. Sr.. Vol. XXXIII, p. 344, 1887. 
Journ 
