MAMMALS OF MINNESOTA. 39 
unfitted for any other life; others climb freely like squirrels; 
again others pass much of their time in the water, and exhibit 
consequent changes in structure; while still other species have 
the elongated hind legs of the deer mice, and are truly terres- 
trial. Organs of sense vary in accordance with the require- 
ments of these various situations. The brain indicates a lower 
condition of the nervous system than in any other group of 
placental mammals. The hemispheres do not cover the cere- 
bellum nor even the corpus bigeminum. The simple structure 
of the brain corresponds to an incompletely developed brain- 
case, for not only is the zygoma often entirely absent, but the 
orbit is never closed, and the whole configuration of the skull 
is upon a primitive type. 
The dentition corresponds to no other group, neither do the 
different genera conform to any common formula. The canines 
are absent (Save in one case) or are replaced by teeth which 
have no resemblance to the canines of other animals. Their 
place is frequently taken by a pair of the incisors. Anatomists 
are not agreed as to the homologies of the various teeth. Fol- 
lowing the incisors are premolars with pointed crowns, and 
following them square crowned molars, which are also armed 
with from three to four saliences. The dentition is well adap- 
ted to the catching and comminution of insects and crustacea. 
The oldest known fossil mammals (from the Triassic) had a 
dentition resembling the Insectivora and, although they may 
have been insectivorous marsupials, the suggestion lies near 
that our present Insectivora are descendants, very little altered, 
of a very numerous group which early separated from the 
common mammalian stem. Undoubted remains of insectivor- 
ous animals occur in the Eocene of America and France, and 
thence onward continue to appear with more and more definite 
affinities tc existing forms. 
The geographical distribution confirms the indications of 
paleontology. The group is widely distributed, but is never- 
theless absent from South America (or nearly so) and also from 
Australia. Hach principal continent has its endemic types 
(families or genera), and even Madagascar has its peculiar 
forms. The Antilles also have their own family of Insectivora. 
Again Europe and Asia support the hedge-hog family, which 
is entirely excluded from America. In the two continents rich 
in marsupials (Australia and South America), there are as yet 
no Insectivora. In places which have been long isolated (Mad- 
agascar and the Antilles), special types have grown up, while 
