DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES. 905 
principle of ‘‘ abbreviation,” the suppression should affect the 
first rather than the second dentition. “‘ Ferner falls wirklich 
jemals eine vollstandige zweite Dentition ausgebildet gewesen 
ware, ist es kaum zu erklaren, wesshalb z.B. bei Didelphys, 
wo durchaus keine Ursache zu einer Reduction oder retro- 
graden Entwicklung des Zahnsystems als Ganzen vorliegt und 
actisch auch keine Reduction eintritt, die zweite und nicht 
vielmehr die erste, im allgemeinen schwachere Dentition un- 
terdriickt wurde—etwas, das ja auch nach dem Princip der 
Abkiirzung der Entwicklung zu erwarten gewesen ware” 
(3, p. 105. See also 28, p. 372 [2)]). 
We note also in this connection the admission of the same 
author, ‘‘ Wo aber sonst innerhalb der Klasse der Siugethiere 
Monophyodontismus auftritt, spricht der zur Zeit vorliegende 
Thatsachenbestand zu Gunsten der Annahme, dass die erste 
Dentition verschwunden ist und die zweite persistirt ” (3, p.143). 
Leche finds in Erinaceus an illustration of this view. He 
has come to the conclusion—we believe justly—that in this 
form those antemolars which are subject to no tooth-change 
are members of the second dentition which have hastened their 
time of development, and that Krinaceus represents “a transi- 
tion form from the diphyodont to the monophyodont stage.” 
Thus suppression in Krinaceus is interpreted in a totally 
different way from that occurring in Marsupials, so that three 
different mammalian conditions may be recognised: (a) the 
lowest or marsupial being characterised by an almost complete 
monophyodontism, from failure to develop successional teeth ; 
(6) the typical mammalian condition, where complete duplicate 
sets of antemolar teeth—milk and successional—are developed ; 
and (c) a still more advanced condition, attained in Erina- 
ceus, Bradypus, and Pinnipedia, which again approaches 
monophyodontism, but this time through defect of the first 
(milk) or historically older dentition. An elaborate hypothesis 
like this, to explain what are essentially very similar con- 
ditions as regards suppression in Hrinaceus, &c., and Mar- 
supials, seems to us to bear its condemnation upon its face. 
We think the proofs are overwhelming that in Marsupials, as 
