599 
known in these groups of the appearance of a dorsal furrow in the 
young. 
Anomaloceras anomalum is a remarkable Silurian fossil, on 
account of the habitual excentric position of the siphuncle, but this 
is always near the venter and in this species the form of the shell 
and character of the sutures show that the genus belongs in the 
same genetic group with Hercoceras. 
In Hercoceras the evidence is very complete that the impressed 
zone originated as a contact furrow. In all the gyroceran forms of 
the allied genus, Ptenoceras, there is nothing of the sort. In the 
loosely coiled forms like Hercoceras trregularis, Pl. viii, Figs. 14 
and 15, there is no dorsal furrow in the nepionic stage. Even in the 
closely allied Hercoceras mirum, although the last has a small um- 
bilical perforation, there was no dorsal furrow in the single specimen 
examined and figured (Pl. viii, Figs. rr and12). So far it is obvious 
that close coiling does not of itself even with a favorable form of 
whorl necessarily bring about the genesis of a dorsal furrow. 
If the sudden bending of a broad whorl was necessarily followed 
by the formation of a dorsal furrow it would certainly have been 
produced in Hercoceras mirum. <A single exception in such cases 
becomes a very significant positive fact against this assumption, and 
that exception appears to occur in thisspecies. The terminal mem- 
ber morphically of this series is Anomaloceras, and in the single 
species of this genus known, there is a dorsal furrow as shown in 
Figs. 16-20, Pl. viii. The umbilical perforation was small in this 
shell, and of course it can be claimed that the furrow in the para- 
nepionic was produced by mechanical pressure, and not inherited 
from forms like Hercoceras, in which it first arose as a contact 
furrow. 
Potoceras dubium, which has been figured on PI. x, Figs. 15-22, 
has unfortunately no recorded locality, but as noted in the description 
there were indications that it was a Devonian fossil. At any rate, 
whatever its age, the characteristics were plain and the presence of a 
dorsal furrow in the paranepionic easily established. 
The length of the ana- and metanepionic substages were decidedly 
Paleozoic, and so also was the large umbilical perforation. It is 
more difficult here to account for the genesis of the dorsal furrow 
upon the mechanical hypothesis than in Anomaloceras on account 
of the large umbilical perforation and the slow growth of the apex. 
Nevertheless it can be reasonably claimed that the abruptness of 
