178 Professor (homy on the [Jan. 67, 



With the exception of one genus, Didelphys, which is American, 

 all the known existing Marsupials laelong to Australia, Tasmania, 

 New Guinea, and its adjacent isles, where the order is represented by 

 the genus Cuscus only. The grazing and browsing Kangaroos are 

 rarely seen abroad in lull daylight, save in dark rainy weather. Most 

 of the Marsupialia are nocturnal. Zoological wanderers in Australia, 

 viewing its plains and scanning its scrubs by broad daylight, are 

 struck by the seeming absence of mammalian life ; but during the brief 

 twilight and dawn, or by the light of the moon, numerous forms are 

 seen to emerge from their hiding-places and illustrate the variety of 

 marsupial life with which many parts of the continent abroad. 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 second type of brain, is exemplified by the mammalia which 

 constitute the order Rodentia, Insectivora, Cheiroptera^ and Bruta : 

 they are grouped together, in the cerebral scheme of classification, 

 under the name Lissencephala,* having reference to the smooth 

 unconvoluted exterior of the brain. 



The RoDENTiA are characterized by two large and long curved 

 incisors in each jaw, separated by a wide interval from the molars ; 

 the teeth being so constructed, and the jaw so articulated, as to effect 

 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 male glands pass peri- 

 odically from the abdomen into a temporary scrotum, and are asso- 

 ciated with prostatic and vesicular 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 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, 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 con- 

 structive instinct in association. In all these inferior physical 

 manifestations we are reminded of Birds. Many Rodents hibernate 

 like Reptiles. They are distributed over all continents. About two- 

 thirds of the known species of Mammalia belong to the Rodent order. 



The transition from the Marsupials to the Rodents, seems to be 

 made by the Wombats ; but the Marsupials graduate more closely by 

 the smaller Opossums to the Insectivora. This term is given to the 

 order of small smooth-brained Mammals, including the Hedgehogs, 

 Moles, and Shrews, the molar teeth of which are bristled with cusps, 

 and are associated with canines and incisors : they are unguiculate, 



* Kic-fxoSj smooth ; kyKc(pc(.Xos, brain. 



