MAMMALIAN ORDERS. 



277 



into a temporary scrotum, and are associated with prostatic and 

 vesicular glands. The placenta is commonly discoid, but is some- 

 times 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 vegetable food.' Certain squirrels achieve short 

 flights by means of expansions of skin between the fore and hind 

 limbs, fig. 154. Some Lemmings perform 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 these inferior psychical manifestations we are 

 reminded of Birds. Many 

 Kodents hibernate like Hep- ^^^ 



tiles. They are distributed 

 over all continents. They 

 have not been found in older 

 deposits than eocene tertiary. 

 The transition from the 

 Marsupials to the Rodents 



is made by the Wombats; 



and the transition from the 



Marsu^oials is made, by an 



equally easy step, through 



the smaller Opossums to the 



Insectivora. This term is 



given to the order of small 



smooth-brained Mammals, the 



molar teeth of which are 



bristled with cusps, fig. 150, 



m, j9, and are associated with 



canines and incisors : they are 



unguiculate, 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 tlie* 



other continents. 



Dentition of a Shrew {Sorex). Magnified. 



