L] ORNITHODELPHIA. 5 



Bandicoots, Dasyures, Thylacines, and Opossums, are the 

 best known representatives of this group. 



III. The Ornithodelphia, equivalent to the order 

 MoNOTREMATA, consist of Only two existing genera; and 

 hitherto, no extinct animals which can be referred to other 

 genera of this remarkable and well-characterized group have 

 been discovered. These two isolated forms, in many re- 

 spects widely dissimilar, yet having numerous common 

 characters which unite them together and distinguish them 

 from the rest of the Mammalia, are the Or?iithorhynchus 

 and the Echidna, both restricted in their geographical range 

 to the Australian continent and the island of Tasmania. 

 Many of the characters in which they differ from the two 

 other sub-classes tend to connect them with the inferior 

 group of Vertebrates, the Sauropsida, especially the Lacer- 

 tians ; for though the name Ornithodelphia owes its origin 

 to the resemblance of the structure of the female repro- 

 ductive organs to those of birds, there is nothing specially 

 bird-hke about them ; all the Sauropsida (Birds and Reptiles) 

 agreeing in these respects. 



The accompanying diagram (page 6) is intended to ex- 

 hibit the relationships which appear to exist between the 

 different groups of the Mammalia. If all known extinct 

 forms were inserted, many of the intervals between the 

 boundary hnes of the groups would be filled up ; other- 

 wise no great modification would be required in their 

 relative position. But our knowledge of the systematic 

 position and relations of the past forms of Mammalian life 

 is in general so imperfect and fragmentary, that it seemed 

 better to confine the diagram to a representation of the 

 present condition of Mammalian life upon the earth. 



