996 The American Naturalist. [December, 
same primary cusp centres? In the lower Eocene the sexi- 
tubercular prototype passes directly back into the tritubercu- 
lar archetype. So throughout the whole mammalian scale 
not only ungulates, but primates, carnivores, insectivores, 
rodents are found playing similar variations upon the primi- 
tive tritubercular type. There are surprisingly few distinct 
types, but an almost unlimited number of sub-types, or varia- 
tions of form. As we descend among the older rocks and the 
various series begin to converge, it becomes increasingly diffi- 
cult to distinguish the different orders by their teeth alone. 
Thus it came about that all the Eocene monkeys were at first 
referred to the ungulates, or to transition groups, as expressed 
in M. Filhol’s composite term Pachy-lémuriens. 
TRITUBERCULAR HOMOLOGIES. 
Embryological Evidence—The progress which has been made 
in the embryology of the teeth is largely in the matter of the 
succession of double series, as indicated by vestiges of earlier 
and later sets of teeth, the so-called milk and permanent sets. 
Embryogenesis, however, has also led to a very minute study 
of the order of succession of the cones in the molar teeth, and 
without entering into the matter in detail, it may be briefly 
stated that all authors are unanimous in describing the cones 
of the lower molar teeth in different groups as developing in 
the same order in which they are supposed to have arisen in 
the past, according to the tritubercular theory, namely: Pro- 
toconid, Paraconid, Metaconid, Hypoconid. In the upper 
teeth, on the other hand, embryogenesis has been found to 
contradict the conclusions reached by the tritubercular theory 
or palingenesis, for all authors have agreed that the order is 
Paracone, Metacone, Protocone, instead of Protocone, Paracone, 
Metacone. When these facts were first brought out by Taeker, 
Rose and others, the writer, with undiminished confidence 1n 
the force of paleontological evidence, advanced as an expla- 
nation the fact that the protocone had become secondarily re- 
duced in the upper molars, and that the embryogeny N°, 
longer recapitulated the order of evolution. This explanation 
has received a measure of support in the latest researches by 
