212 
ON THE STRUCTURE AND CLASSIFICATION 
7 • 1 nni-iLfs also to the Per alest idee, and lends support to 
an exception in internal cusps of the Peredestes type are pro- 
the theory, discussed j^^,,pjatheriideB the crown spreads at the base into the 
duets of the cingulu . Diplocynoelon, or has a median elevation, 
"d as a cusp,^ as in A.yddt,er,.. The internal 
whicn 1 roiessor nremolars of the Jurassic genera, is thus 
or, rven developed and in course of transformatron rate a row of rntema c„ap .» 
3 U, If as now seems probable, the derivation of the mammalian molar from the 
sin.de reptilian cone can be demonstrated by the comparison of a series oOrans.tional 
stages between tire single cone and the three-cone type, and from the alter to the 
central tritnbercular type, the separate history of each cone can certainly be traced 
tlironglioiit the series in its various degrees of modification, development, and depner- 
ntion The remarkable part played by the tritubercular molar has been unfolded by 
the discoveries and writings of Cope. It is undoubtedly the ancestral molar type of 
the Priniatcs, the Carnivora, the Ungulata, the Cheiroptera, the Insectivora, and of sev- 
eral, if not all, of the Marsupialia. For example, we can trace back the quadrituber- 
eiiln'r linnodont, or parent inigulatc type, to the IriUAermlar ■, this to the type with 
three cones in line, which we may call the triconodont type, and this in turn to the hap- 
lodont* reptilian crown. A nomenclature may be suggested for these cones, with reference 
to their order of development and primitive position, to keep clearly before the mind 
their liomologies during secondary changes of form and position. The primitive 
cone inay lie called the protocone ; upon the anterior and posterior slopes of which 
apjK'ar, resjKJCtively, the paracone and meiacone. After the tritubercular crown is 
prcMluced, by the rotation of the lateral cones, inwards in the lower jaw and out- 
wards in tlie uppt'r jaw, the hypocone, or heel, is developed, giving us the tubercular- 
sectorial molar. Exclusive ^ of the Midtituhercidata and of Stereognathus, this is the 
most advanced stage of molar development thus far found in the mesozoic period. 
The protocone of Dromotherium (Plate IX, fig. 17) is prominent and constant 
' Lydckker ha-v" observed a minute inner cusp to the blade of the hinder lower true molars of Amblolhe- 
num, in some cases : (.4 sonciiiuin). Cat. of Foss. Mamm. Part V, p. 274. This apparently is an exception to 
the nilc. 
• " Mcsorxiic Mammalia, p. 14. 
* Wortman (“ Dental Anatomy,” p.418,) writes : “The various steps in this process of dental evolution I 
conceive to have l>ecn as follows; (1) Additions to the anterior and posterior edges of the cone and formation 
of a cingulum. (2) Division of the single root into two. (8) Addition of basal cusps from the cingulum. Long 
continueil vertical pressure, I believe to be an adequate cause for the appearance of the wrinkle or fold of the 
enamel covering at the base of the tooth, which is designated as the cingulum.” These stages, which in part 
had been pointed out by Cope and Allen, coincide remarkably with the actual condition of the molars in 
the Mesozoic Mammalia. 
‘See Cope: ‘‘The homologies and origin of the types of molar teeth of the Mammalia Educabilia.” 
Jour. Phila. Acail., 1874. 
