530 J. T. WILSON AND J. P. HILL. 
it may be an abortive one. And, since the author holds that 
such abortive germs are at least occasionally met with beside 
the true successional teeth of higher mammals, it is evident 
that we are not enabled by their mere presence to 
determine which tooth generation, if any, they re- 
present. 
Elsewhere, however, in a very important passage Leche 
explicitly sums up the argument in favour of the true “suc- 
cessional” character of the lingual downgrowths beside the 
various marsupial teeth in the following words: 
*“‘ Charakteristisch fiir die Beutelthiere sind also sowohl das 
constante Vorkommen und die scharfe Auspragung dieser 
Schmelzkeime und ihre Uebereinstimmung mit dem Schmelz- 
keim des p. 3, als auch ihre lange Permanenz, welche Higen- 
schaften diese Gebilde nicht unwesentlich von den lediglich 
durch die Emancipation der Zahnanlagen von der Schmelzleiste 
entstandenen ‘ Knospen’ unterscheiden, ein Punkt, den ich 
hier ganz besonders betonen méchte. An einigen dieser Schmelz- 
keime sind deutliche Zahnsackchen vorhanden” (38, p. 103). 
Now we cannot but feel that if Leche’s criticisms detailed 
above be well grounded, and if the sceptical lesson he repeatedly 
inculcates have taken hold upon the reader, the latter will find 
it difficult to discover in the summary quoted anything at all 
sufficient to reassure him of the certainty of that identification 
which the author advocates. 
It is difficult to understand how “ constancy of occurrence ” 
—‘‘a sharply stamped character ’’—and “long persistence ” 
can possibly constitute a ‘not unessential” difference. To 
us there appears nothing at all “essential”? about such dis- 
tinctions, and thus far we must hold that Leche’s own verdict 
ought to have been, at most, “ not proven.” 
In one of the passages just quoted from Leche (on p. 108) 
we find him asserting that whenever the free end of the 
(residual) dental lamina is actually swollen in a “kolben” 
or “ knospenformig” manner, and when in addition we can 
recognise the presence of an investment of condensed connective 
tissue to form a “‘ tooth-sac,” then we have a right to speak of 
