406 
larity with which the law of tachygenesis works in producing the 
replacement of hereditary characters in every series of forms, and 
do not trust or know how to use this law. 
The paranepionic substage is consequently among Nautiloids as 
among Ammonoids of longer duration than either of the preceding 
substages and of more variable limits. The siphuncle has acquired 
its ephebic aspect and characters, but it is very often in a different 
position from that which it subsequently assumes, as it is in Vaw- 
tilus pompilius and other forms figured in this memoir. I have 
hitherto considered that it included the latter part of the cyrtoceran 
volution, but it now seems more natural to limit it to that portion 
of the whorl which assumes the gyroceran curve or, in other words, 
turns sharply away from the straighter cone of the preceding sub- 
stages on its return curve towards the apex. This is well shown in 
Mr. Brooks’ drawings and also in the other forms of nautilian 
shells, especially those of Barrandeoceras tyrannum and Sachert 
of the Silurian. At or near the end of the paranepionic substage 
in Mautilus umbilicatus and pompilius there is in almost every shell 
amore or less sharply defined constriction which marks a perma- 
nent aperture. The limits of both substages are subject to varia- 
tions that will be noticed in the succeeding descriptions, but it 
suffices here to note the fact that the upper limits of the paranepionic 
substage are in a general way definable by the limits of the gyro- 
ceran form in close-coiled nautilian shells. That is to say, this 
substage, as a general rule, approaches its end and neanic charac- 
teristics begin to appear at or near the completion of the first volu- 
tion, when growth brings the whorl in contact with the apex or 
dorsal side of the conch. ‘Tachygenic forms are often notable 
exceptions to this definition and introduce modifications that have 
to be studied in each separate series. 
The transformations that distinguish the subdivisions of the 
neanic stage are very well marked in some forms and less distinctly 
in others, but I have constantly found the need of defining two 
stages. Ananeanic is a suitable term for the first substage, which is 
usually well marked in nautilian* shells by the first appearance of 
*In my Genera of Fossil Cephalopods nautilian forms have been defined as those having 
the whorls in such close contact that the dorsum of the enveloping or later formed whorl 
is modified, either flattened or bent inwardly along the area of contact, and has what is 
called an ‘impressed zone.’’ There are, however, some shells that are difficult to classify. 
These have the volutions in contact but do not have an impressed zone. Most of them 
are transitional between gyroceran and nautilian forms and may be placed in either 
category. 
