593 
looseness of the coiling is shown by the free apex and the slight 
development and late incoming of the contact furrow. 
It is, of course, as has been stated above, practically impossible 
in many series to get sufficient evidence to establish the agreement 
of chronology with bioplastology. But there are here and there 
series that show such an agreement, and give approximately com- 
plete and positive evidence in favor of the descent of nautilian from 
arcuate forms. But even if this agreement occurred in a smaller 
number of series than it actually does, the evidence from the 
morphology alone would be sufficient. It is not possible to explain 
why the apex of the transitional forms with large umbilical perfora- 
tions is so often free, or the existence of the larger umbilical 
perforations themselves, or, in fact, any of the peculiarities of the 
nepionic stage, which resemble those of radical forms, except on 
the assumption that they have been derived from these same straight 
or arcuate radicals through direct genetic connection. ‘Thus, 
although the chronological record may coincide with the bioplas- 
tology only in a few series, these few become positive evidence of 
the highest value, that confirms the inferences drawn from the testi- 
mony of the bioplastology and outweighs any amount of negative 
evidence derived from the incompleteness of the record. 
With these remarks, we can now pass on to the consideration of 
the history of the impressed zone, and its mode of origin and 
apparent history in different series. 
There are a number of orthoceran and arcuate forms that may be 
cited as the radicals of the Tarphyceratidee. 
These, like the history of the transitions into Aphetoceras, are 
almost complete, since in this last genus the curvature in the young, 
until a late stage, is so slight that one is not absolutely certain 
whether to consider that such a fragment as is figured on PI. v, 
Figs. 15-17, is really a part of a gyroceran shell or a fragment of a 
cyrtoceran form that never coils. The position of the siphuncle, 
section of the whorl and sutures make the young of these forms 
genetically identical with the adults of such forms as Aphetoceras 
Americanum, and on the other hand the full-grown characters and 
large gyroceran coils, are closer in some species than in others and 
the genus passes by insensible gradations into the more closely 
coiled nautilian genus, Pycnoceras. 
This last has the large umbilical perforation and almost cylindrical 
PROC. AMER. PHILOS. SOC. XXXII. 148. 3W. PRINTED JULY 23, 1894. 
