Hyatt.] 
20 
[May 16, 
great probabilities in favor of the opinion into Agass. Icevigatum 
or striaries , two closely allied species also not infrequently mis- 
taken for Psil. planorbe var. levis. 
The series or genera of these branches not only had a real exis- 
tence as genetic radii arising from Psiloceras, but they can be 
graphically and truthfully presented in tables of which we have 
succeeded in constructing several in different faunas. In each 
basin the taxonomic and chronological relations are in close ac- 
cordance with the classification. The different branches may, there- 
fore, be traced to their natural foci in two different varieties of 
Psiloceras planorbe : the smooth and more degenerate Psil. planorbe 
var. levis , and the pilated variety Psil. planorbe var. plicata. I 
have accordingly divided the group into two natural sub-families 
or stocks, the Levis Stock and the Plicatus Stock ; the Plicatus 
Stock embracing the series of Schlotheimia, Wsehneroceras, Calo- 
ceras and Vermiceras ; and the Levis Stock taking in the series of 
Arnioceras, Coroniceras, Agassiceras, Asteroceras and Oxynoti- 
ceras. 
In each genus or series there are (1) radical forms which approxi- 
mate to Psiloceras ; (2) progressive forms or those which differ 
most from that radical stock ; and (3) degenerate forms. The radi- 
cals have been sufficiently noticed above ; the progressive forms have 
distinctly marked channels, ribs, and spines ; the degenerate forms, 
though not very much altered by degeneration, have smooth shells, 
are apt to lose the channels, and their whorls are more acute on 
the abdomen. The radical, progressive and degenerate forms con- 
stitute regular cycles in each series, and, as has been said in pre- 
vious papers, occur in proper succession in time, the radicals first, 
the progressive species next, and the retrogressive last. These 
relations of the species within the same series is paralleled by the 
relations of the different series when compared together. They 
can also be characterized, as radical series in which degenerate forms 
are rare or more slightly marked ; this is the case in Psiloceras and 
Caloceras. Progressive series are those in which the species are 
very largely progressive in characteristics, like Vermiceras, Arnio- 
ceras, and Coroniceras. Retrogressive series are those in which 
the shells are all-, or in large part, degenerate, such as Agassiceras, 
Asteroceras, and Oxynoticeras. The chronological succession of 
series, like that of the species, within each series, coincides with 
the taxonomical arrangement. » 
