252 OX THE STKUCTUEE AND CLASSIFICATION 
. ,■ •• ,1 , ;(■ this »t*nus is descended from a form with three or four 
fellow, J °Bolodm renders probable, it is again the median in- 
incisors, ns a p ^ evidence bearing upon this point, 
w nt rimlZnee to ascertain whi^i of the incisors is hypertrophied in Pla^ 
^nufax. The lower incisors in PohjmaMon and Plag^a^la^ are reduced to a single 
‘’"'The conine. if present in the previous history of this group, have been entirely 
suppressed in the known forms, with the possible exception of Plag,aidax. 
Premdan. It is interesting to find in the early PkujmuM,.. the typical nura- 
lier of four premolars. There are three premolars in Bolodon, Allodon and Olarox, 
probably two in Trilglodci, and one in Pdymmtodon. There was undoubtedly a 
regular antero-posterior reduction of this series, accompanied in Plagmulax only, by 
the liypertrophy of p,. The tritubercular crowns of in Bolodon are replaced m 
Chirox by three tubercles upon and four upon A hint as to the possible 
derivation’ of the trenchant preinolars of Plagiaulax from tubercular forms, is obtained 
by a study of tlie superior premolars of Ctenacodon serratus (Fig. 8), and comparison 
of the same with the first pair of molars of Bolodon (Plate IX., fig. 16). In the 
former, the trenchant margin is composed of four distinct tubercles; in the latter, 
the internal row of tubercles is partially obliterated by vertical wear of the inferior 
teeth. 
Molars. 'I'he structure of the molars is associated with that of the incisors. 
An almost universal characteristic is the fore and aft grinding motion between the 
upi^r and lower rows of tubercles. Professor Marsh recently called the writer’s 
attention to the wearing of the posterior face of the large upper incisor of Allodon 
hv tlie tip of the lower tooth. The same relation obtains in nearly all the other 
genera, and causes au interference which forces the jaw backwards as it ascends, by a 
mechanism similar to that in the rodents, as demonstrated by Professor Cope.^^ This 
fore and aft grinding motion was found in the Tritylodontidae, Eolodontidae and, later 
Plagiaulacida* ; it is observed in a transition stage from a fore and aft to a circular 
grinding motion in the Polymastodontidte. The most primitive molar crown known 
is that of Microlestes. In this the tubercles are not very numerous and one is much 
more prominent than the remainder (p. 214) ; this inequality is also observed in the 
primitive Plagiaulax niiiior molar, with four tubercles and trace of a cingulum, but 
in tlie more recent genera the tubercles are subequal, the crown is elongate, tuber- 
cles being added posteriorly. In the Tritylodontidae and Bolodontidae the tuber- 
cles are conical. In the Polymastodontidae they are flattened. The transition from 
Ly.lekkcr mentions (Cat. of Foss. Mamin., Part V., p. 195, footnote), that Lemoine describes two upper 
inciaon. and a canine in Xeoplagiaulcu:. I have not met with this description as yet. 
•••The Mechanical Causes of the Origin of the Dentition of the Rodentia.” American Naturalist, 
January, 1888, p. 12. 
