56 Classicfiation oj the Mammalia. 



LlSSENCEPHALA. 



The Lissencephala or smooth-brained Placentals form a group 

 which I consider as equivalent to- the Lyencephala or Implacen- 

 tals ; and which includes the following orders, Hodentia, Insecti- 

 vora, Cheiroptera and Bruta. The Rodentia are characterized 

 by two large and long curved incisors in each jaw, separated by a 

 wide interval from the molars ; and these teeth are so constructed,, 

 and the jaw is so articulated, as to serve in the reduction of the 

 food to small particles by acts of rapid and continued gnawing^ 

 whence the name of the order. The orbits are not separated from 

 the temporal fossae. The testes pass periodically from the abdo- 

 men into a temporary scrotum, and are associated with prostatic 

 and vesicular glands. The placenta is commonly discoid, but i& 

 sometimes a circular mass (Cavy) r or flattened and divided into 

 three or more lobes (Lepus). The Beaver and Capybara are now 

 the giants of the order, which chiefly consists of small, numerous, 

 prolific and diversified unguiculate genera, subsisting wholly or in 

 part on vegetable food. Some Rodents, e. g. the Lemmings, per- 

 form remarkable migrations, the impulse to which, unchecked by 

 dangers or any surmountable obstacles, seems to be mechanical- 

 Many Rodents build very artificial nests, and a few manifest their 

 constructive instinct in association. In all these inferior psychical 

 manifestations we are reminded of Birds. Many Rodents hiber- 

 nate like Reptiles. They are distributed over all continents. 



The transition from the Marsupials to the Rodents is made by 

 the Wombats ; and the transition from the Marsupials is made r 

 by an equally easy step, through the smaller Opossums to the In- 

 sectivora. This term is given to the order of small smooth- 

 brained Mammals, the molar teeth of which are bristled with 

 cusps, and are associated with canines and incisors ; they are un- 

 guiculate, plantigrade, and pentadactyle, and they have complete 

 clavicles. The testes pass periodically from the abdomen into a 

 temporary scrotum, and are associated with large prostatic and 

 vesicular glands : like most other Lissencephala, the Insectivora 

 have a discoid or cup-shaped placenta. Their place and office in 

 South America and Australia are fulfilled by Marsupialia ; but 

 true Insectivora exist in all the other continents. 



The order Cheiroptera, with the exception of the modification 

 of their digits for supporting the large webs that serve as wings,, 

 repeat the chief characters of the Insectivora ; but a few of the 



