OF THE MESOZOIC MAMMALIA. 
243 
thiough the molar series while the para and metacones are irregular in size and 
position, always close to the main cone and in several teeth either splitting into two 
needle-like cusplets or bifid at the tip. Altogether, they are in what appears to he 
an experimental stage of development. Microconodon, however, from the same strata, 
has uell defined para and metacones which are Avidely separated from the main cone, 
the ciown presenting the pure triconodont type. This reoccurs in Ampliilestes, of the 
lower Jurassic, and Triconodon of the upper Jurassic. In this series we are struck by 
the gradual increase of size and prominence of the lateral cones until they are upon 
the level of the main cone and sub-equal to it, this increase being accompanied by a 
marked elongation of the crown so that the three molars of Triconodon occupy a 
greater proportion of the jaw than is taken by the seven molars of Dromotlierium. 
This unmodified triconodont type is very rare in the more recent mammalia. It per- 
sists in the lower jarv, at least, of Dissacus from the Puerco, and in the lower molars 
of Thyktcmus, the upper molars presenting an internal heel. 
b. In his paper upon the Creodonta^ Cope observed that the Spalacotlierium molars 
represent a stage of transition between the triconodont and tritubercular molars. 
There can be no doubt that the cusps seen upon the inner face of the inferior molars 
of this genus are homologous with the para and metacones and there are several facts 
which support Cope’s hypothesis that they represent a stage of inward rotation of 
cusps Avhicli were at an earlier stage in the same fore and aft line with the main cusp. 
These are, that in Phascolotherhim the lateral cones are seen to he slightly internal to 
the main cone so that their median slopes descend upon the inner face ; in Tinodon, 
of a later geological period, this disposition is slightly more pronounced ; in Menaco- 
don it is still more marked but less so than in Spalacotherium. These genera, al- 
though evidently in two different lines of descent, afford the desired transition stages. 
The Spalacotherium molar as seen from above^ has a striking resemblance to the an- 
terior sectorial triangle of the Stypolophus or Didymictis molar of the Puerco. It is 
in fact sub-triangular, the superior molars probably having the lateral cones rotated 
outwards, so that the upper and lower, molars form an alternating series, the ridges 
connecting the main and lateral cones acting as sectorial blades. 
The question now arises whether the Stylacodon molar represents the next 
higher stage of development, viz. the tubercular-sectorial molar in which the anterior 
triangle is followed by a low heel. And if so has the Stylacodon type passed through 
the stages of iiiAvard rotation of the lateral cusps'? The superior aspect of the Styla- 
codon molar presents an anterior triangle Avith the long styloid cone forming the apex 
and connected by divergent ridges Avith the anterior pair of cusps ; behind these is a 
third cusp not connected by a ridge Avith the styloid cone. In the upper jaAv the 
three cusps are external and the single cone internal, these relations are reversed in 
> “The Creodonta,” American Naturalist, 1884, p. 259. 
* Owen. “ The Mesozoic Mammalia.” Plate I, flg. 32c. 
