5.] 103 
[Hyatt. 
This stage is well shown in figures of several species, in the 
Genesis of the Arietidae, p. 10, 11, and in Nautilus ponipilius, in 
Fossil cephalopods of the Mnseum of comparative zoology — Em- 
bryology, V. 3, pi. 3, f. 1, and in a number of figures of Barrande 
in his " Systferae Silurien," pi. 487-488, several of which were 
drawn and given to Barrande by tlie author. I have described 
this first substage among the nautiloids under the descriptive 
name of the " asiphonula," which I now pro])Ose to change into 
Protosiphonula. Among ammonoids this snbstage has been 
forced back into the embryonic stage and has ])ractically dis- 
a|)peared from the conch, probably through the action of tachy- 
genesis. The tendency of the embryo to build a solid calcareous 
protoconch of imbricated structure may be attributed to the 
earlier inheritance of the characteristic's of the calcareous, apical, 
aseptate conch of its nautiloid ancestor. 
This explanation has been supposed by Professor Blake to show 
that the protoconch of ammonoids was necessarily identical with 
the apex of the shell or early part of the ananepionic substage, 
protosiphonula, 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 remnants of protoconchs, covering up the cicatrices of the 
apices of the conch in some nautiloids. These facts must be 
reinvestigated and it must be proved that the latter are not the 
remnants of shrivelled, horny protoconchs, and that the cicatrix 
was not a passage way from the embryo into the shell or at any 
rate an aperture through which the protosiphonula communicated 
with the protoconch, before one can consider them in a different 
light or admit any hyjDothetical explanation. 
It will be seen below that I have altered my view in so far as 
the primary origin and nature of the coecum is concerned. 
Barrande imagined that iny view necessarily implied the passage 
of the embryo bodily out of the protoconch into the conch, but this 
was a mistake arising probably from my inadequate statements. 
The young, Avhen it had passed by growth out of the protoconch 
or when the anterior parts that had grown out of the protoconch 
into this position, began to build the shell, and finally at the end 
of the protosiphonula stage rested in the apex, which was then the 
first living chamber. The structure of the apex in Endoceras, 
Piloceras, and Actinoceras, indicates large and direct, open, 
