DEFINITION OF THE CLASS 



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

 use of in the following brief catalogue of essential mammalian 

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

 hand and the highly specialised Whales on the other. Including 

 those forms, the Mammalia are to be distinguished from all other 

 Vertebrates by 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, be 

 thus denned : 



Hair-clad Vertebrates, with cutaneous glands in the female, 

 secreting milk for the nourishment of the young. Skull without 

 prefrontal, postfrontal, quadrato-jugal, and some other bones, and 

 with two occipital condyles formed entirely by the exoccipitals. 

 Lower jaw composed of dentary bone only, articulating only with 

 the squamosal. Ear bones a chain of three or four separate bonelets. 

 < Vrvical vertebrae sharply distinguished from the dorsals, and if 

 with free ribs, showing no transition between these and the 

 thoracic ribs. Brain with four optic lobes. Lungs and heart 

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

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



The following characters are also very nearly universal, and 

 in any case absolutely 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. 1 

 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 

 spread over a wider area of the earth's surface. As however, though 

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

 birds, they are not quite so widely distributed as are those 

 animals. The Mammalia range up into the extreme north, but, 

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

 known to occur on the Antarctic continent. With the exception 

 of the flying Bats, indigenous mammals are totally absent from 

 New Zealand ; and it seems to be doubtful whether those sup- 

 posed oceanic islands which have a mammalian fauna are really 



1 The degeneration of the hind-limh in "Whales and Sirenia forbids the use of 

 this character as a distinctive one on the principles advocated by the selection of 

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



