1888.] the young Ornithorhynchus paradoxus. 355 



M 



There can be no doubt that these structures are characteristic 

 Mammalian teeth, and their appearance harmonises well with the 

 results of Hertwig's reseai*ches on the structure and development of 

 Placoid scales. His researches indicate that the Mammalian teeth are 

 probably in a more ancestral condition than any other organ possessed 

 by the adult. They must have been derived at one time from Proto- 

 therian ancestors, and yet existing Prototheria were not known to 

 possess them. Their occurrence in Ornithorhynchus, therefore, supplies 

 the step just where it is wanted, and the fact that they are practically 

 identical with the teeth of higher mammals is a further indication of 

 the ancestral nature of these structures, for other higher mammalian 

 features represented in the Prototheria are profoundly modified in the 

 latter. 



Dr. Parker has very kindly placed his material at my disposal, so 

 that I propose to at once investigate, and shall shortly publish a paper 

 upon, the nature of the teeth in the lower jaw of Ornithorhynchus 

 and in Echidna (for it is in every way probable that they will be found 

 in this genus also). I also intend to work at the mature organism, 



ture, although in some sections it has been rendered apparently homogeneous, 

 probably by some method of preparation. — Feb. 6, 1888. 



VOL. XLIII. 2 D 



