442 SCIENCE PROGRESS. 



the development of the teeth in the majority of mammals, 

 and consequently we know next to nothing about the various 

 vestigial teeth, which there is no doubt many of them pos- 

 sess. When Thomas wrote we knew still less, so that it 

 is not surprising to find that many of his generalisations do 

 not hold good. In the case of the incisors I have shown 

 (12) that suppression does not necessarily take place at one 

 end in the series but that it may commence in the middle. 

 His very important discovery (14) of the presence of a milk 

 dentition in Orycteropus carries on the work of Gervais, 

 Flower (25), Tomes (24) and others, and shows that the 

 Edentata are truly diphyodont, and that it is their milk denti- 

 tion which has disappeared. Pouchet and Chabry ( 1 5, p. 173), 

 who had previously worked at the Orycteropus by means of 

 sections, had described the presence near the symphysis of the 

 lower jaw of a minute calcified tooth and an enamel organ 

 of another tooth behind this ; these teeth, which they regard 

 as milk incisors, are anterior to those described by Thomas, 

 and show that these animals originally possessed a complete 

 and probably heterodont dentition. 



Another instance in which the microscope has revealed an 

 important discovery of teeth is that described by Poulton (16) 

 of the true teeth of OmithorhyncJius, afterwards worked 

 out by Thomas (17) and Stewart (18) in older specimens 

 and shown to be functional. It would be of great interest 

 to re-examine Poulton's sections in the light of recent dis- 

 coveries, to determine if possible to which dentition these 

 teeth should be referred. 



Since Rose's discovery ( 1 9) of tooth germs in Manis there 

 only remain the Myrmecopliagidcs and Echidna as possible 

 edentulous mammals, and there can be little doubt that even 

 in these animals we shall ultimately find traces of teeth in 

 the voungr. 



On the continent we find that the milk dentition has long 

 been regarded either as the primitive one, or at least equal 

 in antiquity to the replacing set. The latter view was 

 urged by Lataste (20), who considered that the two sets of 

 teeth were inherited direct from the polyphyodont reptiles, 

 and that the mammalia were primarily diphyodont. 



