240 
ON THE STRUCTURE AND CLASSIFICATION 
iurassic f?euera except Kurtodoa, and is observed upon the onter surface in DqAory. 
nodon It thus in many cases enables us to draw the line between premolars and 
molars, as in both the PeraleatidiP- and in the genera of the Insectivorous Sub-group 
the inner faces of the molars are smooth. The cingulum generally embraces the 
entire inner face of the crown, forming anterior and posterior cingulum cusps or cin. 
(fidesd which are characteristic of the insectivorous forms, while in the supposed car- 
nivorous and omnivorous forms, distinct basal cusps rise posteriorly and sometimes 
anteriorly {Triconoflon) above the cingulum. As in the latter genera the cingu- 
lum is present with the basal cusps, it probably precedes them in evolution, 
but there is no direct ei idence of the conversion of cingules into true basal cusps, 
such as we find in the molars. A review of the premolars of all the genera shows 
that they are sharply distinguished from the incisors and from the molars and less 
distinctly from the canines in many instances. In several genera they have un- 
dergone considerable specialization, as in the production into lofty cones of pm^ 
of Achyrodon or the apparently incipient assumption of the molar pattern in 
Kurtodon. 
Molars. If DromotJierium is a mammal, as there may be some question, it is the 
most reptilian in type of dentition in several respects : first, there is no internal cin- 
giduin upon cither premolars or molars. Second, the premolars have single fangs, w hich 
nniy, by the way, indicate that the division of the fangs in the premolar-molar series 
extended from behind forwards ; third, the division of the molar fangs is incomplete, 
the molars are, strictly speaking, single fanged so far as they are exposed to view; fourth, 
tlie incisors are more reptilian than mammalian in appearance, resembling tho.se of 
uome piscivorous reptiles, or the homodont series of some of the Delpldnklai ; finally 
the molar crowns, although tri- or polycuspid, plainly revert to the monocu.spid con- 
dition. The lateral cusps of the main cone are irregular in size and development 
and appear to present an experimental stage which is transitional between the single 
reptilian cone and the tricuspid, or parent mammalian molar crown, as seen in Micro- 
conodon. 
In the Theromorph reptiles, among which Professor Cope has found many mam- 
malian characteristics, although it is improbable that the mammalia can be derived di- 
rect y from them (Baui-), as already observed upon page 222, we find the teeth im- 
planted in distinct sockets with the bases deeply grooved upon the inner and outer 
aces, {Dimetrodon). The crown is a single cone with a flattened section and serrate 
e( ge, wi 1 no tiace of lateral cones ; from this condition Ave must infer that the pri- 
and nft * u^^ lesulted from some mechanical cause other than the fore 
aft roc king following the production of lateral c ones. In other w^ords the division 
{1876, ^112.)"^ introduced by Harrison Allen. “ Studies in the Facial Region.” Dental Cosmos, 
Ueber die Abstammung der Amnioten Wirbelthiere.” Munich, March 8th, 1887. 
