172 On the Classification of Mammalia. 



order of Edentata is justified by the fact that one genus 

 (Echidna) is completely deprived of teeth, whilst the other 

 (Orniihorhynchus) possesses but a few insignificant ones. 

 These two genera, which constitute by themselves the whole 

 order, may just as well constitute two families, so wide are 

 the differences in their general appearance and structure. 



The Edentata proper constitute a group exceedingly re- 

 markable, composed of a few genera likewise very strange 

 in their characters, strange in their external features, strange 

 in all their relations. The differences amongst these genera 

 are so great that they have been made the types of as many 

 families by systematic writers, and we believe with great 

 propriety. The absence of teeth is the only character by 

 which they are united, although this character is not absolute, 

 inasmuch as grinding teeth in a very rudimentary state are 

 observed in some few : the front teeth or incisors — those 

 never exist in Edentata. Edentata moreover are provided 

 with strong nails or claws to the four locomotory extre- 

 mities. 



Each of the types in Edentata, by its strange appearance, 

 recals to mind another order of things, another physical period 

 in the earth's history, of which they are mere reminiscences. 

 The Tardigrada divide into two groups, one completely ex- 

 tinct, the remains of which are found in the tertiary deposits 

 of South America chiefly, the Tardigrada gravigrada, or 

 Megatheridce ; and another exclusively composed of living 

 representatives, the Tardigrada bradipodida, or sloths of 

 Central and South America. 



§ 2. The order Marsupialia is another combination into 

 one group of strange forms and strange characters, quite as 

 diversified and heterogeneous as in the Edentata, although 

 Marsupialia seem cast upon a more uniform external mould. 

 The great diversity resides in the physiognomy, and in the 

 structure of the teeth. 



In Edentata, we have seen the dentition so defective, than 

 in several cases teeth were entirely absent. Here in Mar- 

 supialia the dentition is greatly developed, becomes a perma- 

 nent character, and requires a contrasting importance. The 

 incisors, it is true, are nowhere six in each jaw, which is the 



