1897,] `: Trituberculy : 995 
accompanying table of terms used among the rhinoceroses and 
horses alone. It could not have been anticipated that the 
diverse molars of the horse 
and of the rhinoceros, for 
example, would be limited 
in their variations, in a late 
geological period, by their 
unity of origin in an ex- 
tremely early geological 
period. Yet. such is un- 
doubtedly the case. Com- 
pare the accompanying 
figures of Merychippus and 
nr a of Aceratherium. Imagine 
_ Fig. 2.—RutNoceros Morar. Unde- that you see the simple 
I 8 y folds. bunodont molar of such a 
form as Deen's Hyracotherium vulpiceps, underlying these 
diverse crests and crescents. Consult Taeker’s “ Zur Kenntniss 
der Odontogenese bei Ungulaten ” and you will find that this 
sexitubercular archetype is not imaginary, but is a constantly 
recurring fact of embryonic development—all the crests and 
crescents being preceded in the embryo by simple cones. Then 
compare carefully the variations in the two teeth as follows: 
The two “cement lakes” of Merychippus with the two “ fos- 
settes” of Aceratherium, enclosed in the former by crescentic 
spurs, and in the latter by the “ antecrochet” and “ crochet ;” 
seas} 
postfossette 
\ 
\ 
perastyle-<— protoconid Aypooonid, 
protoconule ~- tee ~hypoconulid 
protocone ~ ~~ 5 
` mataconid’ entoconid: 
Fig. 8. SSNS BUNODONT Morar. —Hyracotherium vulpiceps, after Owen. 
the posterior “lake” and “ fossette ” similarly enclosed. by 
an upgrowth of the posterior. basal cingulum. Can any.one 
question the homologies between these secondary adaptations 
to a diet of grasses when it is seen that they spring from the 
