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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 amtnonoids. 

 Thus the first septum and caecum 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 caecum 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 

 caecum, 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 Endoceratidae. 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 genesiol-ogy 

 as a working hypothesis, looks for the largest representation of an- 

 cestral characters in the earliest and most generalized forms. Thus 

 the Goniatitinae 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 



