412 
the apex of the shell in Nautiloidea. This and the fact that the 
protoconch is calcareous are in favor of the opinion that the charac- 
teristics of the ananepionic substage of the ancestral nautiloids ap- 
peared in combination with the protoconchial stage in ammonoids, 
Thus the first septum and czecum in this order is the floor of the 
first living chamber of the apex of the conch and is one substage 
earlier in this order than in nautiloids, and should be called anane- 
pionic. | 
The figures, so far as the shell is concerned, also seem to demon- 
strate that the czecum at this substage probably represents some em- 
bryonic structure. ‘This is Zittel’s explanation of the origin of the 
siphuncle, it being as stated by him obviously traceable to the 
cecum, and this in turn being probably formed out of a part of the 
body or the shrunken mantle of the embryo, since it lies in the 
Ammonoidea directly in the aperture of the protoconch. 
While, however, this organ fills the diameter of the apex in the 
median plane, it is narrower laterally, and one feels that this sup- 
position is open to certain objections that will be discussed more 
fully in a paper now in preparation on the Endoceratide. It may 
be mentioned here, however, that in these ancient forms of the 
Nautiloidea the opening from the siphuncle into the protoconchial 
shell is closed in a different way from what it is in the normal 
Nautiloidea, and in the protosiphonula the endosiphuncle communi- 
cated with the protoconchial shell, passing through the bottom of 
the caecum and apex. ‘The elements of the walls of the siphuncle 
surrounding the endosiphuncle in these forms are, however, similar 
to what they are in the Nautiloids of less primitive organization, 
and it becomes probable that the caecum was formed in the meta- 
nepionic substage in Nautiloidea as a secondary epembryonic organ, 
and that this has been crowded out of the metanepionic into the 
ananepionic in Ammonoids. In other words, like some other char- 
acters it was acquired in the epembryonic stages of Diphragmo-. 
ceras and like these has been inherited earlier in descendants. 
One naturally, if disposed to adopt the theories of genesiology 
as a working hypothesis, looks for the largest representation of an- 
cestra] characters in the earliest and most generalized forms. Thus 
the Goniatitinz of the Silurian, which belong in all except the 
terminal members of series like Pinnacites and Celceras to this 
category, one ought to find the transitions to Bactrites, or, failing 
these, indications in the young of the less specialized forms of the 
