ON- THE KTKUCTUKE AND CLASSIFICATION 
, ■ , We cannot well avoid the infeieuce that the SylaoodoA lower molar 
the lower jaw . that the styloid external cusp, which until 
J. a si-ialired 
Marsh's internal cusps represent the paracone and meta- 
protocone while • , the hvDOcone or heel. This is further confirmed 
cone followed J f Spalaeotlierium type seen in the molars of Adkeiwdon 
cot while the remainder of the crown is closely 
Itr lo that of St.la^on. The internal cusps present many degrees of develop- 
nicnt in different members of the StuUKoMda, ; in Laodo«. toy are much less 
Ziinent than in Z>rj,o/c.<«, the heel being also inconspicuous While the relations 
of the four cones composing the Stylacodont crown strongly suggest the tuber- 
cula, -sectorial molar there is one matter of doubt in the way of the derivation 
of this tooth from the Spatacolliermm type; that is, the position of the fangs. 
In SjHihicothennm and Menacodon the fangs are paired and placed beneath the 
pam and metacones. In the Stylacodonts the external fang is directly beneath the 
protorone; the question is does this represent the anterior or posterior, or an addi- 
tional fangl . 
4.° The molars which have been considered thus far show directly or indirectly 
the triconodont type, i. e., the presence at some stage of their evolution of the central 
and two lateral cones. In the Amphitheriidee it is clear that the main cone and the 
lc8.scr one, upon its anterior slope, represent the protocone and paracone but it is un- 
certain whether the basal cusp, seen for example upon the external face of the Diplo- 
cyuodon molar, is homologous with the metacone or hypocone. The latter alterna- 
tive excludes the development of the metacone or the passage of these genera through a 
triconodont stage and implies a considerable separation of the Avipliitheriida; from the 
stem of the two families already considered. The former involves the supposition that 
the metneone has metamorphosed into a heel. The most primitive molar in this fam- 
ily is seen in Etmeodon^ The crown has an obtuse recurved protocone, more like 
that of a preniolar ; ujion the anterior slope is a rudimentary paracone which affords 
the only means of distinguishing the molars from the premolars. The posterior slope 
terminates in a low extended heel. This molar pattern largely confirms the second of 
the above alternatives, viz, that this heel is to be compared to the hypocone of the 
tubercular-sectorial crown. Further confirmation is seen in the fact that this heel 
is not above the level of the internal cingulum, as in the metacone of all the 
triconodonts, but is continuous with the broad shelf-like projection of the internal 
cingulum, which is well represented in the internal aspect of the DipJocynodon 
molars. The concave internal slope of the protocone descends into this shelf and 
the cingulum rises at the margin into numerous crenations, which cannot properly 
• Marsh, “.Vmeric.an Jurassic Mammals.” Am. Jour. Sc., April 1887. PI. X, fig. 4. 
