DEFINITION OF THE CLASS 



acters might he used in addition to those which will he made 

 use of in the followinj^ hrief catalogue of essential mammalian 

 features, were it not for the low-placed Monotremata on the one 

 hand and the highly specialised Whales ou the other. Including 

 those forms, the Mammalia are to l)e distinguished from all other 

 Vertehrates hy the following series of structural features, which 

 will be expanded later into a short disquisition upon the general 

 structure of the Mammalia. The class Mammalia may, in fact, Ite 

 thus defined : — • 



Hair-clad Vertehrates, with cutaneous glands in tlie female, 

 secreting milk for the nourishment of the young. Skull without 

 prefrontal, postfrontal, ([uadrato-jugal, and some other hones, and 

 with two occipital condyles formed entirely by the exoccipitals. 

 Lower jaw composed of dentary bone only, articulating only with 

 the squamosal. Ear l)ones a chain of three or four separate l)onelets. 

 Cervical vertebrae sharply distinguished from the dorsals, and if 

 with free ri)>s, showing no transition between these and the 

 thoracic ribs. Brain with four o])tic lobes. Lungs and heart 

 separated from abdominal cavity 1 )y a muscular diaphragm. Heart 

 with a single left aortic arch. lied blood-corpuscles non-nucleate. 



The following characters are also very nearly universal, and 

 in any case al)solutely distinctive : — Cervical vertebrae, seven ; 

 vertebrae with epiphyses. Ankle-joint " cruro-tarsal," i.e. be- 

 tween the leg and the ankle, and not in the middle of the ankle.^ 

 Attachment of the pelvis to the vertebral column pre-acetabular 

 in position. 



The Mammalia since they are hot-blooded creatures are more 

 independent of temperature than reptiles ; they are thus found 

 s])read over a wider area of the earth's surface. As liowever, though 

 hot-blooded, they have not the powers of locomotion possessed by 

 birds, they are not (piite so widely distributed as are those 

 animals. The Mammalia range up into the extreme ncn'th, but, 

 excepting only forms mainly aquatic, such as the Sea Lions, are not 

 known to occur on the Antarctic continent. With the exception 

 (jf the flying Bats, indigenous mammals are totally alxsent from 

 New Zealand ; and it seems to l)e doubtful whether those sup- 

 posed oceanic islands which have a mammalian fauna are really 



^ 'J'lie degeneration of the liiinl-liiul) in Wliales and Sin-nia fofbids the use of 

 this character as a distinctive one on the principles advocated l)y the selection of 

 the above list. But it would be absurd to leave out hair. 



