22 PEOEESSOE OWEN ON THE CHABACTEBS, ETC. 



emerge from their hiding-places and illustrate the variety of mar- 

 supial life with which many parts of the continent abound. "We 

 may associate with their low position in the mammalian scale the 

 prevalent habit amongst the Marsupialia of limiting the exercise 

 of the faculties of active life to the period when they are shielded 

 by the obscurity of night. 



The Lissencephala or smooth-brained' Placentals form a group 

 which I consider as equivalent to the Lyencephala or Implacentals ; 

 and which includes the following orders, Bodentia^ Insectivora, 

 Cheiroptera and Bruta. The Eodentia 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 tem- 

 poral fossas. The testes pass periodically from the abdomen into 

 a temporary scrotum, and are associated with prostatic and vesi- 

 cular glands. The placenta is commonly discoid, but is sometimes 

 a circular mass (Cavy), 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 vege- 

 table food. Some E-odents, e. g. the Lemmings, perform remarkable 

 migrations, the impulse to which, unchecked by dangers or any 

 surmountable obstacles, seems to be mechanical. Many Eodents 

 build very artificial nests, and a few manifest their constructive 

 instinct in association. In all these inferior psychical manifesta- 

 tions we are reminded of Birds. Many Eodents hibernate like 

 Eep tiles. They are distributed over all continents. 



The transition from the Marsupials to the Eodents is made by 

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

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

 Insectiyoba. 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 

 unguiculate, plantigrade, and pentadactyle, and they have com- 

 plete clavicles. The testes pass periodically from the abdomen 

 into a temporary scrotum, and are associated with large pro- 

 static and vesicular glands : like most other lAssencephala, the 

 Insectivora have a discoid or cup-shaped placenta. Their place 

 and office in South America and Australia are fulfilled by Marsu- 

 pialia ; but true Insectivora exist in all the other continents. 



