May, 1893. SUCCESSION OF TEETH IN MAMMALS. 361 
these arguments, and in April of last year an authority as well 
qualified to speak on the subject as Mr. Oldfield Thomas, declared in 
favour of a diphyodont condition being the most primitive one in 
Mammalia—a condition, moreover, which was probably derived from 
the polyphyodontism of Reptilia. 
The first argument drawn from the Odontocetes, the toothed 
whales, is shattered by the discovery of an embryonic series of teeth 
within those which later pierce the gum to persist as the teeth of the 
adult. In an advanced embryo of a dolphin (Phocena communis), 
possessed of the twenty-five teeth in each mandible, rudiments 
of a second series were found internal to these. Kiikenthal 
accordingly pronounces the teeth which are present throughout the 
life of these whales as representatives of the deciduous teeth of other 
Mammalia, while the uncut rudiments would correspond with the 
adult dentition of higher forms. These rudiments are, although small, 
perfectly recognisable as teeth, and are possessed of a cap of enamel 
and an inner core of pulp. 
It was the Marsupials, however, that supplied the chief argu- 
ments to Sir W. H. Flower and Mr. Oldfield Thomas as to the 
secondary character of the diphyodont condition in Mammalia, and 
the most important part of Kikenthal’s work on this subject is, 
therefore, the discovery of an almost complete series of successional 
teeth in some of the opossums. 
In his classical paper of 1887, Mr. Oldfield Thomas,? after a con- 
siderable discussion of the teeth of Marsupials, arrived at the 
conclusion that originally all Mammalia were wholly monophyodont, 
the permanent teeth having ancestral priority over their ontogenetic 
predecessors of the deciduous series. 
To account for the supposed secondary development of the 
deciduous dentition, it was assumed that a retardation of the per- 
manent teeth would be beneficial by preventing overcrowding before 
the mouth was fully grown, while at the same time some provision of 
masticatory organs for a young animal with its teeth thus retarded 
would be essential. This advantageous retarding of the teeth, 
together with the necessity of finding some temporary grinding 
apparatus, was supposed to have resulted in the formation of a 
deciduous set of teeth which are of greater or less complexity, and 
retained for a longer or shorter time in different members of the 
Mammalia. 
From the fact that only one tooth is replaced in the Marsupials, 
it was further supposed that the Mammalia had reached a fairly high 
degree of complexity before the necessity for deciduous teeth became 
urgent, since they could only have possessed one deciduous tooth 
when they had already gained all the complications of a generalised 
Metatherian condition. It was considered that at this juncture the 
2 Philosophical Transactions of the Royal Society, 1887. 
