1002. The American Naturalist. [December, 
increasing support for the writer’s hypothesis, that the multi- 
tuberculate tooth is of tritubercular origin. 
2. THE EARLY STAGES OF SEXITUBERCULY. 
The Trigon.—Respect for Cope’s priority should not prevent 
our ultimately adopting the late Professor Riitimeyer’s term 
trigonodont for the third stage, retaining the term “ tritu- 
tubercular” as descriptive of the whole transformation, and as 
peculiarly appropriate to certain types of teeth, such as the 
superior molars of the lemurs. “ Trigonodont ” is most appro- 
priate because the first step in molar morphology is to identify 
the “primitive triangle,’ and the term “tubercular” hardly 
applies to a lofty pointed cutting crown. Our studies among 
the Mesozoic mammals have left no doubt that the upper and 
lower triangles, or “ trigon ” and “ trigonid,” were derived from 
the reptilian protocone by the addition of lateral cusps. The 
mechanical perfection of this type consisted in the fact that the 
lateral cusps were developed upon or shifted to the outer side 
in the upper molars, and to the inner side in the lower molars, 
thus producing an interlocking “shear.” The “trigon” was 
essentially a cutting apparatus, so perfect that many mam- 
mals retained it without further evoluiton. Thus Chrysochloris, 
the little Insectivore of the Cape, presents a fine example o 
this type, persistent in its molars. (See Figure 4). 
The Talon.—But in the great majority of trituberculates the 
“talon” was added as a crushing apparatus. It invariably 
appeared first in the lower molars (where we may distinguish 
it as the “ talonid ”) and pressed into the basin of the superior 
“trigon.” At first it was a mere spur (hypocone) as in Amphi- 
therium or in the existing Calcochloris (allied to Chrysochloris), 
but between the Jurassic and Upper Cretaceous periods the 
talonid widened into a basin-like shelf supporting an outer 
cusp, the “hypoconid ;” an intermediate cusp, the “ hypocon- 
ulid,” and an inner cusp, the “entoconid.” Thus we find 
in the majority the Upper Cretaceous (Laramie) and Puerco or 
lowest Eocene mammals that the lower molars bear six cusps; 
the above-mentioned three on the talonid and three on the 
trigonid (protoconid, paraconid, metaconid). With these six 
