ANAGENESIS. 71 
ieee 
GENESIS OF CHARACTERISTICS. 
ANAGENESIS,’ OR THE GENESIS OF PROGRESSIVE CHARACTERISTICS. 
HE introduction of the peculiar pile and single channelled abdomen in 
Schlotheimia, which occurred in the earlier species, must be regarded 
as a progressive complication, since it is not only a new characteristic, but it is 
correlative with constant progress in the amount of involution, and with the 
advent of species having more compressed, more involute and sub-acute whorls. 
The weehneroceran series introduces the observer to Schlotheimia by its inter- 
mediate modifications when one begins with the radical stock, Psiloceras. 
Undoubtedly the steady increase of involution in successive species of the 
psiloceran, wehneroceran, and schlotheimian series is, like the similar phenomena 
of other series, to be regarded as progressive. This is clearly shown, both by 
the steady increase of size in individuals throughout each series, and also by 
the fact that these changes are in accord with the general progression of the 
whole group. 
The younger stages, as we have remarked above, were closer coiled during 
the Mesozoic than in the Paleozoic, and the adult forms were more involute as 
a rule in the Trias and Jura, than in the earlier geologic periods. The genesis 
of this progressive character independently in each series may be seen by ex- 
amining the different series in the four summary plates, and we need not allude 
to it again in this chapter. It is also interesting to note, that the most exact 
parallelisms in this respect are to be found between the series, which are 
widely separated. Thus the extreme aberrant forms of the family, namely, 
Schlotheimia, Weehneroceras, the radical Psiloceras, and the opposite extremes 
of the group, Asteroceras and Oxynoticeras, all possessed highly involute 
compressed whorls. 
Neither of the first three had any species with quadragonal whorls; the shells 
are all modifications of the helmet-shaped or secondary radical form. There is 
an approximation to the subquadragonal in the most discoidal species, Sehdot. 
catenata and some others, but this is not very noticeable, and the absence of 
geniculs in the pile confirm this conclusion. These series, therefore, can be 
placed in strong contrast with the more normal species of the caloceran series, in 
which the quadragonal form of whorl and its correlated characters played a promi- 
nent part. The acmic species of the progressive series of the Arietidae were 
these pilated and tuberculated quadragonal whorls. In the development of the 
individual, also, the quadragonal form and tubercles are the last characters added 
by progressive growth, and were primarily of ephebolic origin. 
1 Avd, upwards ; Téveous, descent by birth. 
