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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 irregularis , PI. viii, Figs. 14 

 and 15, there is no dorsal furrow in the nepionic stage. Even in the 

 closely allied Hercoceras miritm, although the last has a small um- 

 bilical perforation, there was no dorsal furrow in the single specimen 

 examined and figured (PI. viii, Figs. 11 and 12). 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 this species. 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, PI. 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 dubiut?i, 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 



