ON THE STUUOTUKE AND CLASSIFICATION 
246 
• .V,- .,,,1, of the molars the following are the principal deductions ; 
Hcvicwing this study ol th , y „oup presents one mam cusp 
(.) The molars of all the 1 may be considered homologous with 
Ihich is either so central or so prom.nent that . t may 
e,.e single reptilia., upm' Jre rterior and^posterior slopes of 
cusi>s, the para and me . ’ PP frequently repeated stage of evolution. It 
H.e protocone. Th.s .s a “tloplnt ; in L first, the para- and meta- 
gives rise to two lines persistent triconodont type, 
conesareretaiued n the^fo- and^aft ^ ^ 
but increase grea y in . , In a second line of genera the 
hcrciilar type, which ^ y jijg protocone but the metacone is not 
or HypLone which .tends Inwards to 
form °lm intema”l cusp, (d) In a fourth line of genera neither the pp- nor me^- 
concs are developed upon the sides of the protocone, but they are replaced by. basal 
cusps derived from the cingulum. 
Reduction and Succession of the Teeth. 
The homologies of the molar cusps naturally have an intimate bearing upon the 
pliylogcnctic problems, i. e., of the relations of these families of mammals to each 
otlier and to a common primitive stock. It must be constantly kept in mind, how- 
ever, tlint like meclmnical or functional forces produce like effects, so that we may 
nlmo.st as.sunie that the triconodont and tritubercular type has appeared independently 
iu widely different phyla. To counteract errors which may arise from this law of 
development, valuable data are afforded by a comparison of the dental series as a 
whole, in the genera embraced in the different mesozoic families, with respect to 
the retardation, atrophy, suppression, acceleration^ and hypertrophy of the teeth. 
These terms arc here employed to express, first, the relatively late time of ap- 
pearance of a tooth in its adult position; second, the relative decrease in size 
of a tooth as (compared with its fellows of the same series ; third, the loss or absence 
of a tooth ; fourtli, the relatively early time of appearance ; finally, the increase 
of size from excess of nutrition. Thus retardation and acceleration, atrophy and 
' Coi)« has employed ‘‘acceleration” in a larger sense as expressing an increase or addition of parts as 
well ns an increase of rate of growth (Proc. Phila. Acad., 1876, p. 16). But in the dental series, as lately 
observeil l>y Oldflehl Thomas (loc. cit., pp. 452-3), an increase of size is frequently preceded by a relative 
decrease or retardation in the rate of growth ; the term must be here used in the more restricted sense. 
Kowalevsky has employed •' reduction ” in all his memoirs to express the process of loss of one of a series of 
teeth or limb members, and this term has now come into universal use. We may describe a dental series as 
reilnccd. i. e., from the typical complement of the teeth, in which one or more teeth have been suppressed. 
Atrophy is frequently used as equivalent to “ suppression,” but may better retain its original significance. As 
all changes result from a transfer of nutrition they may be described as metatrophic. 
