Classification. - The class Mammalia was divided by Linnaeus into three principal sections, Unguiculata, Ungulata and Mutica. The last comprises the cetaceans, the second all the un gulates except the elephant, and the first, the remainder of the class. This classification was replaced by Blainville, who proposed on embry ological grounds to divide the class into Mono delphia, or mammals with a placenta ; Didel phia, or mammals without a placenta (the marsupials) and Ornithodelphia, or the mono tremes. Richard Owen combined the last two subclasses under the name of Eplacentalia and gave the placental mammals the name of Plo centalia. Speculation as to the origin of the class as a whole led Huxley to propose as the source a hypothetical group which be named Hypotheria, the characters assigned being the absence of milk glands and of a corpus callo sum in the brain and the presence of a quadrate bone for the articulation of the mandible. Ex isting mammals were divided into Prototheria, comprising the monotremes, Metatheria, the marsupials, and Eutheria, the so-called placen tal mammals. Cope in 1889, while retaining the subclass Prototheria for the monotremes, placed the entire remainder of the class in the subclass Flower and Lydekker (1891) adopt Huxley's divisions, while Bed dard (1902) makes use of those of Cope. Flower and Lydekker's arrangement of families and higher groups is as follows (fossil groups printed in italics): The groups of existing mammals whose structural peculiarities are such as to entitle them to rank as separate families vary greatly as regards the number of genera and species they comprise, some being represented by a mul titude of different forms, while others consist only of a single species, or a single genus with but a few species. Families consisting of only
a single genus and species are as follows: The Chirompdm, established for the reception of the Aye-Aye, a singularly modified lemuroid mam mal, confined to Madagascar; the Dinomyidm, comprising only a large Peruvian rodent, some what like a of which a single specimen is known; the Antilocapridx, represented only by the Prong-horn of the western plains of North America; the Notbryctidx, comprising only a small mole-like marsupial recently discovered in South Australia; the Ornithorhynchidx, com prising only the Platypus, or Duck-bill, of Aus tralia. These and other restricted families are to be looked upon as fragments of groups of genera and species, of which the greater num ber are extinct, or as branches from main lines of development which have never progressed and ramified.
F. F.., 'Mammalia' (London 1909) ; Hornaday, W. T., 'American Natural History' (New York 1910) ; Kingsley, J. L.,