12 GENESIS OF THE ARIETIDA. 
Nautilus, and in the vast majority of all known fossils of the order so far known 
to the author this stage had similar characters. The siphon was larger at this 
stage than subsequently, and possessed a prolongation which reached down into 
and lined the primitive cecum. ‘This closed pipe was however more or less cylin- 
drical, and formed a transition to a cylindrical, open, siphonal tube, when com- 
pared with the caecum on the one hand and the siphon of the succeeding septa 
on the other. The second septum was prolonged apically into a funnel, and this 
was continuous with a true porous wall, which formed the remainder of the 
pouch. We have already pointed out the probability that this wall was the 
homologue of the calcareous sheaths or endocones which filled the interiors of 
the siphons of Endoceratide. There is, therefore, as previously stated by the 
author, a structural though highly concentrated and much modified remnant of 
the adult siphonal elements of an Endoceras still preserved in this stage, even in 
the existing Nautilus, and we propose to name it the Macrosiphonula. 
9. The next neepionic stage in living Nautilus was the third living chamber 
and third septum with its siphon. The siphon has a true funnel, and the siphonal 
wall attached to it is less swollen out, and seems, upon re-examination of the . 
junctions at the opening of the funnel in the second septum, to be discontinuous. 
If we are correct, this stage has a small siphon consisting of the usual funnel and 
tubular porous wall, as in the vast majority of all Nautiloidsand Ammonoids. We 
proposed to name this, according to its aspect and structure, the Microsiphonula. 
The microsiphonula, though a nepionic stage in the modern Nautilus, did: not 
always occur among larval stages, but had in common with the macrosiphonula 
a traceable beginning in the adult stages of ancestral types. The genesis of the 
two forms of hic may be studied in the Endoceratide. In this family Cyrto- 
cerina had a siphon, which continually increased in size, probably throughout life, 
though more forms need to be described before one can be assured a this as a 
fact. There is no doubt, however, that the next form, Piloceras, had what we can 
safely call a macrosiphon of typical structure until very late in life. The large 
shells collected in Newfoundland by the author had siphons of great. size, aiak 
were only slightly contracted or remotely approximated to the tubular condition 
of the Orthoceratide in the adolescent and adult stages. 
This and other changes occurring in the adolescent stages induced us to 
distinguish them by a special term, Nealogic. The adolescent or nealogic 
stages, therefore, and the stages of the adult, or, as we have named them, the 
EKphebolic stages, in Piloperas show for the first time a tendency to contract 
the siphon or approximate to the microsiphonula, but they never had a true 
microsiphon. The contracted siphon in these forms, as in the other genera of 
this family, always had the holochoanoidal or complete funnel reaching from one 
septum to the next, and a series of conical concentric endocones, or sheaths, as 
they have been called by others, which stretched from the ends of the funnels, 
and were the homologues of the porous walls of the segments of the siphon in 
Nautilus. 
The terminations of the endocones were prolonged into a central tube. or 
endosiphon, which we have previously described, and which probably served as a 
i 
