ANAGENESIS. 71 



III. 



GENESIS OF CHAEACTERISTICS. 

 Anagenesis, 1 or the Genesis of Progressive Characteristics. 



THE introduction of the peculiar pilae 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 wcehneroceran 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, wsehneroceran, 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, Wsehneroceras, 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, Schlot. 

 catenate/, and some others, but this is not very noticeable, and the absence of 

 geniculse in the pilas 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 Arietidaa 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 'Ara, upwards ; Two-ir, descent by birth. 



