104 
[Api-il s, 
tiiUiil.u- (•oiinerLion hetweeii tlic |»rot()concli :ui<l the animal when 
ill tliis first cliainber througli which the endosiphuncle in tlie 
gciiorali/A'd nantiloids, Eiidosiphonoidea, opened into the proto- 
roiu-h. 
'V\\v tiibuhir opening of the apex in Endoceras, Piloceras, and 
Aetinoceras, and other genera having a marked endosipliuncle, is 
not closed by the coecum of the siphuncle as was formerly 
sup|»osed. It is on the contrary directly continuous with tlie 
endosiphuncle, as was first pointed out by Foord in his •• Catalogue 
of British Cephalopoda." This is an attenuated, central, more or 
less irregular tube formed by the extension ot" the successive 
endocones or sheaths. It is more or less interrupted by pseudo- 
septa, and is a separate and distinct organ occupying the axis of 
the lai'ge siphuncle. This organ is continuous with some corre- 
sponding part, possibly the mantle of the embryo, which existed 
ill the protoconch. On the other hand, the true siphuncle, 
including the coecum of the first air chamber, is a secondary organ 
formed by the funnels of the septa. 
The next substage is indicated by the presence of the coecum 
Iving within the apex, and this is formed by the funnel of the first 
septum and in association with the first septum is universal among 
Ce})halopoda, with the exception of some sejjioids, so far as tlie 
internal structures are concerned. It has been descriptively 
named the coecosiphoiiula. This may be considered as a part of 
the metanepionic in nautiloids, but among ammonoids and 
belemnoids it is forced back according to the law of tachygen- 
esis into the ananepionic substage, the calcareous apex of the 
ancestral shell being consolidated with and disappearing in the 
calcareous protoconch of these two orders. 
The limits of the living chamber which rested upon this first 
septum have not been determined, Init in a general way it may be 
said that some of the external characteristics of this age ai-e also 
characteristic of most of the Nautiloidea. The shell is c3a-toceran 
or curved, the septa succeeding the first septum, that may in 
different forms be said to belong to this stage, are entire, never 
have annular lobes on the dorsum, and always have saddles on the 
venter and dorsum, or are straight. The markings of the external 
surface are, however, variable and in respect to the latter and to 
the outline of a section of the whorl at this time the characteristics 
cannot be considered as of ordinal value. 
