May. 1893. SUCCESSION OF TEETH IN MAMMALS. 361 



these arguments, and in April of last year an authority as well 

 quahfied 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 {Phocwna 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 wath 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 Kukenthal'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 compHcations of a generalised 

 Metatherian condition. It was considered that at this juncture the 



2 Philosophical Transactions of the Royal Society, 1S87. 



