403 
Cephalopods of the Museum of Comparative Zodlogy, ‘* Embryol- 
ogy,’? Vol. iii, Pl. iii, Fig. 1, and in a number of figures of Bar-' 
rande in his Systéme Silurien, Pls. 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. 
~ Tt 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 
