PART III: INTRODUCTION. 779 
(Proc. Zool. Soc. London, 1897), and the arrangement of the Marsu- 
pialia is that of the same author, as given in the ‘ Catalogue of Marsupia- 
lia of the British Museum’ (1888), with a few necessary modifications. 
In the extinct families Abderitidee, Epanorthidee, and Garzonide, 
the recent arrangement of Ameghino has been followed (Anal. Mus. 
Nac. Buenos Aires, IX, p. 159, 1903). In the case of the Ungulata 
the following arrangements have been adopted: That of Sclater and 
Thomas for the Antelopes, that of Lydekker for the other Bovidee 
and the Cervide, and those of Osborn, Matthew, and Earle for some of 
the extinct groups of North America. 
Under this treatment it will be interesting to note the number of 
families which are monotypic—that is, comprise only a single valid 
genus. Seventeen such families are recognized by Trouessart,^ while 
18 are here recognized, as shown in the following list: 
Allotheria: Marsupialia: 
Tritylodontidze. Notoryctide. 
Edentata: Paurodontidee. 
Orophodontide. Monotremata: : 
Fer: Ornithorhynchide. 
Protelide. Primates: 
Glires: Daubentoniid:e. 
Aplodontiide. Hominidee. 
Dinomyide. Tarsiidee. 
Lophiomyide. Sirenia: 
Pedetide. Hydrodamalide. 
[nsectivora: Prorastomid:e. 
Chrysochloridee. Ungulata: 
Galeopithecid:e. Antilocapridz. 
Solenodontidee. 
NOMENCLATURE. 
In the designations of the higher groups discrepancies will often be 
noticed upon comparison with the nomenclature used in other works 
of reference. The name of the class Mammalia is one of the few 
names concerning which there is universal agreement. For sub- 
classes two sets of terms are in common use—Ornithodelphia, Didel- 
phia, and Monodelphia of De Blainville, and Prototheria, Metatheria, 
and Eutheria of later authors, which are given preference in some 
recent works.” These terms, however, are not properly synonymous 

4 TROUESSART gives the Trichechid:e, Procaviide, Pyrotherid:e, Pantolambdid:e, and 
Polyma:todontid:e in addition to the groups above mentioned, but unites some of the 
remaining families with other groups, or places additional genera under them, so that 
they are not monoty pic. 
b **The Eutheria may embrace the Meteutheria or Marsupials, the Meseutheria or 
primitive Mesozoic Placentals, the Ceneutheria or Tertiary Placentals."  (OsnoRN, 
Am. Journ. Sci, 4th ser., VII, p. 93 footnote, Feb., 1899.) The last two groups 
were previously called Mesoplacentalia and Cenoplacentalia (OssoRwN, Trans. N. Y. 
Acad. Sci., XIII, pp. 234—237, June 4, 1894). 
