DIGESTIVE ORGANS. 215 
Not infrequently the enamel is lacking from the teeth of mammals, as in whales, 
dugongs and edentates, or it may be restricted to one side of a tooth,’ as in the 
incisors of rodents. Sexual differentiations occasionally occur in mammals, certain 
teeth (usually canines or incisors, more rarely premolars) being better developed 
in the males than in the females of the same species. 
There are two views as to the way in which the complicated molars of the mam- 
mals have arisen. Both start with the conical tooth as the primitive condition. 
One theory is that the fusion of such simple teeth is sufficient to account for the 
multiplication of roots and tubercles in all of their varying forms (figs. 217, 218). 
> 
> 
a 
>> >>>> Ss) 
BED >> S>>> >>>) 
=_) exj>>>>>5>>4 
>>> >> Sal 
>>> > a > sw 
>>>>>T SS 
a>>>> 
oR 
ef Das >> >> >>5 
=) Sxj>>>>55 > 54 
=) =—js> s>= >> oa 
=) Ge Jaess>>>>5 
= 
=> 
>> 
ES 
MAMAN 
= ] 
3 \NAA 
ASAANIAANAM Anant Ansin AAAI ALALALAT A BA ff Wdsnslsanns lannadanrnnn lang Ancil NAa 
Fic. 217.—Diagram of the relation of the human teeth to the primitive dentition, after 
Rose. 
The other hypothesis is that parts have been developed on the primitive cone, 
giving, first, the triconodont shape. Next these three cones have been shifted to 
the tritubercular position; and later other parts—hypocone, lophs, etc.—have 
been added and these have been modified in different directions. Each view has 
much in its favor. Embryology is not at all decisive, while paleontology favors the 
latter view. 
Epidermal Teeth occur in cyclostomes and in larval amphibia and 
in embryonic monotremes. In the cyclostomes they are cones of 
cornified epithelium covering an underlying core of the integument; they 
are differently arranged in the lampreys and myxinoids. In the latter 
they are few, there being a single tooth on the ‘palate’ and two chev- 
ron-shaped rows on the tongue. In the lampreys nearly the whole 
inner surface of the oral hood is lined with these teeth of varying shape, 
