400 
MR. O. THOMAS ON THE HOMOLOGIES AND 
years are so well known as to need no more than a passing mention from me. In all 
questions of fact, especially in connection with the fossil forms, I have gained great 
assistance from them, an assistance for which I must make my due acknowledgment. 
Of Professor Flower’s papers, already referred to, it need only be here remarked that 
every opinion he expressed has been fully confirmed, and that any advance on his 
papers is due to the examination of a far larger series of specimens than were available 
to him—an examination carried out very largely on the lines indicated by him. To 
Mr. C. S. Tomes’s invaluable work, ‘ Dental Anatomy,’ I am also largely indebted for 
information in regard to the growth and early development of the teeth. 
Of the more recent foreign contributions to the subject, the most important are 
those of Professor E. D. Cope'"' and of Dr. It. BaumeI ; but the differences between 
their views and those now brought forward are so considerable, and involve so much 
detailed argument, that a criticism upon them would here be out of place. It must, 
therefore, suffice to say that them respective views on the descent and homologies of 
teeth have been fully taken into consideration during the preparation of the present 
paper. 
In conclusion, as it is to the general advantage that true theories should be con¬ 
firmed and untrue ones soon exploded, I have thought it useful to draw up a few 
notes on the possible or probable discoveries relating to this subject, in order that 
they may be looked for and their true bearing understood by persons interested in, 
and having opportunities for making observations on, tooth-homologies. 
1. The discovery in a recent Marsupial of a milk-tooth preceding one of the per¬ 
manent set other than pm 4 , and especially i 1 . This discovery, although unlikely to be 
made, would on the whole be confirmatory of the views above advocated, as it would 
show that the process of the formation of milk-teeth is still going on in the Marsupials 
on the lines believed to have been followed by the common ancestors both of them and 
of the Eutheria, 
2. The same in a fossil, undoubtedly Marsupial, and in its other characters allied to, 
and perhaps ancestral to, the living forms. This would be obviously entirely fatal, as 
it would show that the view as to the Marsupial tooth-change being a remnant and 
not a commencement of a full change is, after all, the true one. On the other hand, 
it is just possible that some of the extinct Marsupials may have antedated the existing 
species in the formation of a fuller milk dentition, and have then died out from some 
unexplained cause. A full, and not a rudimentary, tooth-change in a fossil Marsupial 
would, therefore, be the best and most final disproof of my views. That such a dis¬ 
covery, however, will ever be made, I cannot believe, especially considering the 
astonishing persistence of precisely the same amount of tooth-change from the 
Mesozoic to the modern Marsupials. 
* Papers in tlie ‘ American Naturalist,’ ‘ Proceedings of the American Philosophical Society,’ and 
elsewhere. 
f ‘ Odontologische Forschungen.—Versueh einer Entwickelungsgeschichte des Gehisses,’ 1882. 
