498 The American Naturalist. [June, 
well as in Macropus. In other cases the last premolar of the 
first series is absorbed, and in its place enters the premolar of 
the second series; this more primitive type is found in an 
undetermined species of the genus Phalangista ; also in Macro- 
pus lugens and M. gigantius; also, according to the figures of 
Oldfield Thomas, in Phascogale, and in the fossil Triacantho- 
don serrula. 
“ Further researches are necessary to determine which type is 
the most common among the Marsupials. It also remains to 
ascertain through sections and models, whether the last incisor 
(fifth or lateral, i°) of the upper jaw really arises from the sec- 
ond series, and in what species this occurs. Possibly in some 
Marsupials other teeth also rise from the second series. 
Whether this, however, is the case or not, the principal theory 
of Marsupial dentition is not thereby affected. It is certainly 
well established by my researches, as well as by those of 
Kikenthal, that the teeth of the Marsupials, with the excep- 
tion of the last premolar and probably of the last superior 
incisor of some species, belong to the first series, and are 
analogous with the milk teeth of man and other mammalia. 
“ This shows that in the entire vertebrate series the principle 
holds good, that by the better development of the single tooth, 
the frequent succession of teeth of the Selachian type, is 
gradually limited. The Marsupials, however, in the reduction 
of the multiple succession of the reptilian-like ancestors of 
existing mammals, have together gone past the limit, and 
have wandered into a cul de sac of evolution out of which there 
was no return. Upon this rests the remarkable constancy 
of this order, from the Mesozoic period to the present time. 
The reduction of the former multiple succession of teeth of the 
vetrebates to a single series of teeth, appears therefore, only to 
be of service when the teeth at the same time attain a perma- i 
nent growth. This stage was only attained among the 
Marsupials by Phascolomys.” 
In the same paper the author restates his theory of the 
origin of premolars and molars by the fusion of several single - 
teeth, and is led far astray from the actual conditions which we 
observe in such development. Originally, he says, the pre- 
