TRUE TEETH AND HORNY PLATES OP ORNITHORHYNCHUS. 21 
embryology of the group/’ Mr. Thomas, in Plate xxviii, 
accompanying his paper, indicates the same argument in a 
diagram, which shows the Edentate dentition as a side off- 
shoot arising low down from the generalised Prototherian level 
of the main Proto-meta-eutherian stem. 
In the same connection, the present paper bears in an 
important manner upon Tomes’s discovery of an enamel oi’gan 
in the developing teeth of toothed Edentata (in Armadillo, 
1. c. 128, 282, ‘ Quart. Journ. Micro. Sci.,’ 1874, and ‘ Phil. 
Trans./ 1876). 
Although Tomes did not consider that the presence of the 
enamel organ proved that enamel had been previously formed 
in the ancestors of the group, the observation is manifestly 
consistent with such an interpretation, which is further sup- 
ported by the conclusions of Oldfield Thomas ; and now that 
an enamel organ and enamel have been proved to exist 
in a living representative of those ancient mammals from 
which the Edentates arose, there appears to be little doubt 
about the significance of Tomes’s important discovery. In this 
statement I am only referring to the existence of an enamel- 
organ in Edentata, and express no opinion as to the universal 
presence of this structure independently of the later produc- 
tion of enamel. In order to come to safe conclusions upon 
this latter fact, it will be necessary to study the development 
of teeth more widely than has yet been possible throughout 
the Vertebrate sub-kingdom. 
Again, the structure and development of the rudimentary 
teeth of Oruithorhynchus strongly confirm the opinions of the 
many writers who hold that teeth are in a more ancestral 
condition than perhaps any other structure possessed by the 
adult mammal. While the other higher mammalian organs 
and structures represented in the Monotremes are profoundly 
modified in the latter, the teeth remain practically identical in 
form, structure, and development. We have only to compare 
the structure of the skeleton or ovary of a Monotreme with 
that of any other mammal in order to realise how' much the 
identity of the dental structures proves for the excessively 
