2 1 L ZOOLOGY. 



may have points of resemblance so close to two groups as to 

 render it almost indiH'nvnt to which it be referred. To 

 some naturalists, dillerenees appear important which an- dis- 

 regarded by others, and hence a want of agreement on the 

 subject of classification has always prevailed. 



The method followed here is nearly that proposed by 

 Cuvier. It rests mainly on the differences mammals show 

 in respect of their extremities and teeth, differences which 

 ;il ways imply a crowd of others in structures, habits, and 

 even intelligence. 



410. Keeping in view the ensemble of these characters, 

 the class mammalia may be divided into two groups, the 

 monodelphic and didelphic. 



The monodelphic or monodelphian are the more numerous, 

 and are distinguished chiefly by their mode of development. 

 At birth they are already provided with all their organs, and 

 before birth they derive their nourishment from the mother 

 by means of a placenta. Their brain is more perfect than 

 the didelphian, by the presence of a corpus callosum uniting 

 the two cerebral hemispheres ( 186). Finally, the walls of 

 the abdomen have no osseous supports attached to the margins 

 of the pelvis, as we find in the second great class of mam- 

 mals. The mammals thus organized have been subdivided 

 into two groups, namely, ordinary mammals and pisciform 

 mammals. 



411. The ordinary mammals are organized principally 

 to live on solid ground; the skin is provided with hairs. 

 These animals are further subdivided into ten orders : the 

 bimana, quadrumana, cheiroptera, insectivora, rodent i; 

 tata, carnivora, amphibia, pachydermata, and ruminantia. 

 The first eight of these orders have flexible fingers and toes, 

 with nails covering only the dorsal aspect of the toe or 

 finger, and comparatively small ; hence they have been called 

 onguiculata; the last two, namely, the pachvdrnnata and 

 ruminantia, have the extremity of the finger and toe entirely 

 enclosed in a hoof; they are thus called ongulata. 



412. The order bimana includes only man : in him alone 

 the arms arc de.-tined tor prehension, the limbs for progiv>sion 

 and support in the erect attitude. Thus, the natural position 

 on the soil is unmistakeably vertical. The teeth are of three 

 kinds ( 52), and have their edges on the same plane : they arc 

 fragivaroiu; finally, the brain is more perfect, more highly 

 developed, than in any other animal. 



