744 
It is a striking fact that all the molars of the triassic and juras- 
sic periods are distinguished by one conic cusp much larger and more 
prominent than the others which are smaller and upon a lower level. 
What are the positions and homologies of this cusp in the upper and 
lower teeth? 
2° That the protocone is invariably the anterior 
lateral (antero-external) cusp in the lower molars and 
the anterior lingual (antero-internal) cusp in the upper 
molars. The former part of this proposition is now almost self- 
evident. It is absolutely proven in such a series as we see in 
Figure 1 and is now corroborated by the embryological researches of 
TAEKER and RÖse. 
As to the present position or homologue of the reptilian protocone 
in the upper mammalian molar there is relatively, I admit, more room 
for doubt, mainly for the reason that fossil upper jaws are very Scarce. 
If, as held by FLEISCHMANN and Ross, the antero-external cusp 
is the protocone then the whole system of homologies held by CoPpE 
and myself falls to the ground. Let us look at the evidence: 
First: In the numerous upper jaws of Triconodon of the 
upper jurassic, the main cusp is always the middle one of the three, 
corresponding with the large middle cusp of the lower molars which 
we know to be the protocone, Second: In the upper molars of 
Spalacotherium (jurassic) in which the lower molars are of the 
simplest tritubercular type (Fig. 1, 4) the most prominent cusp by 
far is the antero-internal, supporting my view. Third: In all 
the Amblotheriidae of the upper jurassic there is a triangle of 
cusps in both upper and lower molars, in each the apex is formed by 
the most prominent styliform cusp, this is antero-externalin 
the lower molars and antero-internal in the upper 
molars. Is it at all probable, at this early period, when the pro- 
toconid is still the most conspicuous cusp in the lower molars that 
a corresponding cusp of the same form, but reversed position, in- 
variably found in the upper molars is not homologous? According 
to the FLEISCHMANN-ROSE view it is not, but the main lower cusp 
is homologous with one of the spurs of the main upper cusp. Fourth: 
There are other important grounds of a mechanical nature. Starting 
with the study of modern, instead of the oldest fossil forms, FLEISCH- 
MANN has, I believe reached not only an erroneous conception of the 
homologies of the separate cusps but of the equally important homo- 
logies in the functional regions of the upper and lower crowns. In 
each we may distinguish two regions: 
