Geology and Paleontology. 927 
that the tritubercular stage, in one form or other, is universal 
among the known lower Eocence Mammalia. In a recent memoir, 
I showed that a large proportion of the Mammalia of the Mesozoic 
period, again excluding the Multituberculates, were in the line of 
trituberculy, and a renewed examination of the English types 
removes every one of the apparent exceptions to this law. Among 
the American Jurassic types there are still several apparent excep- 
tions. ; 
In view of the evidence for the almost universal presence of the 
tritubercular stage in the present or past history of the upper and 
lower molars, I have already advocated a distinct nomenclature for 
the different cusps which compose this molar and its derivatives, up 
to the stage of the acquisition of six tubercles in the upper molars 
and five in the lower. This is the final stage in which the 
tubercles reman distinct. The nomenclature now in general 
recog 
nized at once that the antero-internal cusp of the lower molar of 
Mioclænus is not homologous with the antero-internal cusp of the 
upper molar of the same genus, nor is it homologous with the 
antero-internal cusp of the lower molar of Hyopsodus. 
e nomenclature proposed is based upon the fact that the 
cusps composing the main triangles are homologous with each other 
. and that some of the cusps superadded to these to form respectively 
the six and five tubercled mı slars, have probably originated in a 
similar manner. The terms for the three main cusps are selected 
to indicate, as far as possible, the primitive position and the order of 
evolution. The lower molar cusps are arbitrarily distinguished 
rom those of the upper molars by the termination id. 
Terms proposed. Terms now in use. 
Up. Molars. Low. Molars. Upper Molars. Lower Molars. 
Protocone, Protoconid. Antero-internal cusp. Antero external cusp. 
Paracone. Paraconid. Antero-external “ Antero-internal “ 
$ cusp. 
Metacone. Metaconid. Postero-external ‘‘ Diahsen iniaa Me 
or intermediate ‘ 
Hypocone. Hypoconid. Postero-internal ‘ Postero-external “ 
cusp. 
Protoconule. Anterior-interme 
diate cusp. 
Metaconule. Posterior-interme- 
diate. 
Epiconid Postero-internal cusp. 
This note is from an abstract of a paper presented to the British 
Association at Bath upon the Evolution of the Mammalian Molar 
