180 PROF. W. H. FLOWER ON THE ARRANGEMENT [Apt. 17, 



which has just appeared in the XVth vohirae of the ' Encyclopaedia,' 

 the groups will be found more fully defined than it is necessary to 

 do here ; but it was suggested to nie by our Secretary, that it would 

 be desirable to place before the Fellows of the Society, in a more 

 convenient form, an abstract of the arrangement adopted, preceded 

 by a few explanatory notes upon the mutual relations of some of the 

 principal groups. 



One of the most certain and fundamental points in the classifi- 

 cation of the Mammalia is, that all the animals now composing the 

 class can be grouped primarily into three natural divisions, which, 

 presenting very marked differential characters, and having no exist- 

 ing, or yet certainly demonstrated extinct, intermediate or trans- 

 itional forms, may be considered as subclasses of equal value, tax- 

 onomically speaking, though very different in the numbers and 

 importance of the animals at present composing them. These three 

 groups are often called by the names originally proposed for them 

 by Blainville — (1) Omithodelphia, {2) Bidelphia, (3) Monodelphia — 

 the first being equivalent to the order Monotremata, the second to 

 the Marsupialia, and the third including all the remaining members 

 of the class. Although actual palaeontological proof is wanting, there 

 is much reason to believe that each of these, as now existing, are 

 survivors of distinct branches to which the earliest forms of Mammals 

 have successively given rise, and for which hypothetical branches 

 Professor Huxley has proposed the names of Protoiheria, Metatheria, 

 and Eutheria ^ names which, being far less open to objection than 

 those of Blainville, are here used as equivalents of the latter. 



The only known Prototheria, though agreeingin many important 

 characters, evidently represent two very diverging stocks, perhaps as 

 far removed as are the members of some of the accepted orders of the 

 Eutheria. It would, however, be encumbering zoological science 

 with new names to give them any other than the ordinarily known 

 family designations of OrnithorhynchidcB and Echidnidce. 



Similarly with regard to the Metatheria. although the great 

 diversity in external form, in anatomical characters, and in mode 

 of life of the various animals of this section might lead to their 

 division into groups equivalent to the orders of the Eutheria, I do 

 not think it advisable to depart from the usual custom of treating 

 them all as forming one order, called Marsujnalia, the limits of 

 which are equivalent to those of the subclass, and the primary divi- 

 sions of which are called " families." The hmits of these six fami- 

 lies are extremely well marked and easily defined ; and as they form 

 a regular gradation between two extreme types, they can be satis- 

 factorily arranged in a serial order. 



The remaining Mammals are included in the Eutheria, Placen- 

 talia or MoNODELPHiA. Their affinities with one another are 

 so complex that it is impossible to arrange them serially with any 

 regard to natural affinities. Indeed each order is now so isolated 

 that it is almost impossible to say what its affinities are ; and none 

 of the hitherto proposed associations of the orders into larger groups 



' P. Z. S. 1880, p. 649. 



