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Cephalopods of the Museum of Comparative Zoology, "Embryol- 

 ogy/' Vol. iii, PL iii, Fig. i, and in a number of figures of Bar- 

 rande in his Sysieme Silurien, Pis. 487, 488, a few of which were 

 drawn and given to Barrande by the author. I first described this 

 substage among the nautiloids under the descriptive name of the 

 "asiphonula," but have since substituted the term, Protosipho- 

 nula. Among ammonoids this substage has been forced back into 

 the embryonic stage and has practically disappeared from the conch, 

 probably through the action of tachygenesis. The tendency of 

 the embryo to build a solid calcareous protoconch of imbricated 

 structure may be attributed to the earlier inheritance of the char- 

 acteristics of the calcareous, apical conch of its nautiloid ancestor. 



This explanation has been supposed by Prof. Blake to show that 

 the protoconch of ammonoids was necessarily identical with the 

 apex of the shell or early part of the ananepionic substage, proto- 

 siphonula, of nautiloids. It would have such a meaning, perhaps, 

 if there were a cicatrix on the protoconch of ammonoids and if 

 there were not more or less rugose lumps, supposed to be the rem- 

 nants of protoconchs, covering up the cicatrices of the apices of 

 the conch in some nautiloids as figured above on page 360 of the 

 Introduction. These facts must be reinvestigated by the opponents 

 of this view, and it lies with them to prove that the latter are not 

 the remnants of shriveled, horny protoconchs, and that the cicatrix 

 was not a passageway from the embryo into the shell or at any rate 

 an aperture through which the animal of the protosiphonula com- 

 municated with the protoconch, before one can consider the facts 

 in a different light or admit any other hypothetical explanation. 



It will be seen below that I have altered my view in so far as the 

 primary origin and nature of the caecum is concerned. Barrande 

 imagined that my view necessarily implied the passage of the em- 

 bryo bodily out of the protoconch into the conch, but this was a 

 mistake arising probably from inadequate statements. The young, 

 when it had passed by growth out of the protoconch, or as the an- 

 terior parts of the embryo grew out of the protoconch into this 

 position, began to build the shell, and finally at the end of the pro- 

 tosiphonula stage rested in the apex, which was then aseptate and 

 was the first living chamber. The structure of the apex in Endo- 

 ceras, Piloceras and Actinoceras indicates large and direct, open, 

 tubular connection between the protoconch and the animal when 

 in this first chamber through which the endosiphuncle in the 



