DO INDEX GENERUM MAMMALIUM. 
such by its author; (2) the first species referred to the genus, or the 
species standing first on the page if no type is designated explicitly or 
otherwise; (3) in the case of Linnean genera, the best known European 
or * officina! species of the genus; but in case of doubt, the first spe- 
cies; (4) the species, if any, which has furnished the name of the genus, 
provided it be mentioned by the author of the genus; (5) in case of old 
generic names restricted by common consent to a species not the first 
mentioned by the author, such species may be accepted as the type 
to avoid confusion, provided the restriction antedates any modern 
names for the same genus. 
As already stated, an attempt was first made in the preparation of 
this index to fix the type of each genus, but afterwards abandoned in 
favor of the expedient of enumerating all the species included in the 
genus by the original describer and marking the one indicated as the 
type by a subsequent reviser of the group. 
The types of practically all the genera proposed by Linnzeus have 
been fixed by modern workers: those published by Brisson have been 
fixed by Merriam; those adopted by Kerr and Oken have been deter- 
mined by Allen;’ those proposed by Gloger have been fixed by 
Thomas;^ those of marsupials and monotremes have also been given 
by Thomas; those of antelopes by Sclater and Thomas;’ those of 
existing genera of South African mammals by W. L. Sclater;/ those 
of recent North American genera by Merriam, Allen, Miller, Bangs, 
and Bailey: those of most of the extinct genera by O. P. Hay;% and 
many scattered types have been fixed by specialists. Thus the type 
species of the great majority of existing genera are now determined 
with some degree of precision. 
HYPOTHETICAL GENERA. 
The term ‘hypothetical genera’ is here used to include genera which 
are based on characters assumed to have been possessed by mammals 
still unknown. | Probably in no other branch of zoology has this device 
of completing the paleontological record been so freely used. Nearly 
fifty hypothetical genera of mammals have been published during the 
last fifteen years, chiefly by Cope, Haeckel, and Ameghino. These 
genera have been proposed to fill eaps in certain groups or to indicate 
the generalized types from which known genera are supposed to have 
been derived. Several were intended to close the gap between man 


5 Bull. Am. Mus. Nat. Hist., New York, VII, 1895; XVI, 1902. 
^ Ànn. and Mag. Nat. Hist., 6th ser., XV, Feb., 1895. 
“Cat. Marsupialia and Monotremata Brit. Mus., 1888. 
* Book of Antelopes, 1896-1901. 
J Mamm. S. Africa, 1900-1901. 
7 Cat. Fossil Vertebrates N. Am., Bull. 179, U. S. Geol. Surv., 1902. 
