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 th'e 

 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 Prance, and 

 thence onward continue to appear with more and more definite 

 affinities to 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. Each 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 grownup, while 



