1883.] OK THE ORDKRS AND FAMILIES OF MAMMALIA. 181 



stand the test of critical investigation. All serial arrangements of the 

 orders are therefore perfectly arbitrary ; and although it would be of 

 very great convenience for reference in books and museums if some 

 general sequence, such as that here proposed, were generally adopted, 

 such a result can scarcely be expected, as equally good reasons might 

 be given for almost any other combination of the various elements 

 of which the series is composed. In fact I have already seen reason 

 to depart in some respects from that used in the ' Encyclopaedia.' 



The Edentata, Sirenia, and Cetacea stand apart from all the 

 rest in the fact that their dentition does not conform to the general 

 heterodont, diphyodont type to which that of all other Eutheria 

 can be reduced, and which is such a close bond of union between 

 them. In all three orders, however, some indications may be 

 traced of relationship, however distant, with the general type. 



I must refer to a paper communicated to the Society last year for 

 my views as to the grouping of the animals composing the Edentata, 

 which diifer from those of most, if not all, zoologists who have pre- 

 viously made them their study'. I there gave reasons for believing 

 that the Sloths and Anteaters were nearly related, and that the Arma- 

 dillos, though much modified, belonged to the same stock, but that 

 the Pangolins and the Orycteropus each represented very isolated 

 forms. The division of the order into four suborders here proposed 

 is an attempt to represent these views, though not altogether satis- 

 factory, as the present divergence between the first two families is 

 scarcely sufficiently indicated by tlieir association in one suborder. 



There is no difficulty about the limits of the order Sirenia, com- 

 posed of aquatic, vegetable-eating animals, with complete absence of 

 hind limbs, and low cerebral organization, represented in our present 

 state of knowledge by but two existing genera, Halicore and Manu- 

 tus, and a few extinct forms, which, though approaching a more 

 generalized mammalian type, show no special characters allying them 

 to any of the other orders. The few facts as yet collected relating 

 to the former history of the Sirenia leave us as much in the dark 

 as to the origin and affinities of this peculiar group of animals as 

 we were when we only knew the living members. They lend no 

 countenance to their association with the Cetacea ; and, on the other 

 hand, their supposed affinity with the Ungulata receives no very 

 material support from them. 



Another equally well-marked and equally isolated, though far 

 more numerously represented and diversified order, is that of the 

 Cetacea, placed simply for convenience next to the Sirenia ; for 

 except in their fish-like adaptation to aquatic life they have little 

 in common with them. The old association of these orders in one 

 group can only be maintained either in ignorance of their structure 

 or in an avowedly artificial system. Among the existing members 

 of the order, there are two very distinct types, the toothed Whales 

 or Odontoceti, and the Baleen Whales or Mystacoceti, which pre- 

 sent as many marked distinguishing structural characters as are 

 found between many other divisions of the Mammalia which are 



1 P. Z. S. 1882, p. 358. 



