104 INTRODUCTION TO CLASSIFICATION. 



are freely articulated with one another, and the phalanges do 

 not exceed three. 



There is only one set of molar teeth, and horny plates are 

 developed upon the premaxillary region of the palate and the 

 opposed surface of the lower jaw. The apex of the heart is 

 deeply bifid between the two ventricles. 



The existing Sirenia are the estuarine, or littoral, Dugongs 

 and Manatees. 



In the remaining order, the EDENTATA, the placentation 

 appears to vary, being diffuse and non-deciduate in Manis, 

 cotyledonous (and non-deciduate ?) in Bradypus, and discoidal 

 and deciduate in Oryeieropus ; but further investigation is needed 

 before such variations can be safely admitted to exist. 



The teeth are always devoid of enamel and of complete 

 roots. There are never any median incisors in either jaw, 

 and incisors are entirely absent in all but one genus of Arma- 

 dillos. 



The fore-limbs are well-developed, and their ungual phalanges 

 are enveloped in long and strong claws. There are pectoral, and 

 sometimes abdominal, mammae. 



The Sloths, the extinct Megatherium and its allies, the 

 Anteaters, the Pangolins, and the Armadillos belong to this 

 order. 



The charactersof the orders MARSUPIALIA and MONOTREMATA 

 are the same as those of the sub-classes Didelphia and Ornitho- 

 delphia, of which they respectively constitute the sole members. 



II. THE SAUROPSIDA. 



The class AVES is divisible into three orders 



1. SAURUR^:. The metacarpal bones are not ankylosed 

 together. The caudal vertebra are both numerous and large, 

 so that the caudal region of the spine is longer than the body. 



This order contains only the extinct bird, Arehseopteryx. 



2. KATIT^E. The metacarpal bones are ankylosed together. 

 The tail is shorter than the body. 



The sternum is devoid of any crest and ossifies only from 

 lateral and paired centres. 



