44.8 J. T. WILSON AND J. P. HILL. 
respected in variation.”” It will be noted that these conten- 
tions do not necessarily affect our belief that the normal teeth 
are individually homologous in different forms; but only 
lead to the belief that the facts of variation do not yield reli- 
able evidence in favour of such an homology in any given case, 
If this be so, then the first and a very important support for 
Thomas’s theory gives way. 
Even less weight can be attached to the second considera- 
tion adduced in support of Thomas’stheory. Rose does indeed 
mention a possible rudiment of Thomas’s “ pm. 2” in Didel- 
phys. But for this Kikenthal sought in vain, and, as a 
result of a later and more extended research on Didelphys 
and other marsupial forms, Leche formulates his judgment 
upon the matter in the following passage: 
“‘TIn diesem Zusammenhange mochte ich ausdricklich her- 
vorheben, dass die ontogenetischen Untersuchungen bisher 
keinen Aufschluss tiber die Homologien der einzelnen Zahne 
der Beuteltiere und derjenigen der Placentalier gegeben habe. 
Auch die von Thomas versuchte Homologisirung der Pramo- 
laren der Beuteltiere gewinnt durch die ontogenetischen 
Befunde keine Stiitze ” (8, p. 107). 
Woodward, however, has recorded his discovery in Ma- 
cropus giganteus (2, p. 463) of “numerous small enlarge- 
ments and irregularities ” in the diastema between the canine 
and the “third” premolar of the upper jaw, “some of which 
may possibly represent the missing premolars.” This is vague 
enough. But he goes on to say, ‘In the lower jaw, however, 
there is a very distinct vestige of a tooth in the form of an 
irregular enamel-organ with enamel epithelium and pulp (fig. 
14). This, from its proximity to the third premolar, must re- 
present pm. 2.” Since, however, there is another typical 
marsupial premolar to be accounted for somewhere in this 
situation, it seems to us not a little rash to pronounce the 
enamel-organ in question—merely on account of its proximity 
to p. 83—to be “pm. 2,” a tooth admitted to be otherwise 
entirely absent from the marsupial dentition. 
Like Kiikenthal and Leche, we have sought in vain for any 
