610 
The impressed zone is among most of the Ammonoidea therefore 
essentially a contact furrow, and the tendency to close coiling has 
been accelerated to.so great an extent that contact takes place 
between the permanent protoconch and the ananepionic substage and 
a contact furrow is thus produced earlier than in any known Nauti- 
loid. The position of the first septum in the aperture of the pro- 
toconch shows that contact must have taken place before it was 
deposited as the floor of the ananepionic living chamber, Zz. e., at 
the very beginning of the building of the apex of the conch. 
It is also obvious that this high degree of acceleration in devel- 
opment was attained in the Devonian, as a permanent hereditary 
character of the whole order since the Nautilinidz are the only 
representatives of the Goniatitinz in the Silurian and disappear in 
the Devonian. ‘There are also but very few species with open um- 
bilical perforations in the Devonian, outside of the Nautilinide, 
and so few in the Carboniferous, that Branco denies the correctness 
of my figures of the two species above mentioned. ‘That open um- 
bilical perforations should occur sporadically in the young of some 
Carboniferous species of Goniatitine is of course to be expected, 
and that Branco should not have found any simply demonstrates 
the rarity of their occurrence. 
The history of the impressed zone among Ammonoidea is par- 
allel with that of the Nautiloidea in regard to the shell layers 
on the dorsum. ‘These are complete in Bactrites and all of the 
Nautilinidge which do not have a contact furrow and incomplete in 
all Ammonoids that do have this furrow, the outer layer reaching 
only to the lines of involution. This is shown in Fig. 3, Pl. iii, 
and it # observable in this that the shell of the apex of the conch 
appears to end at the outer edges of the umbilical perforations, but 
this observation needs revision or confismation. There is a third 
layer between the dorsal and ventral walls of the shell correspond- 
ing to the organic black layer of Nautilus and it is often calcareous 
and well preserved in some fossils.* 
The extraordinary variety of degenerative series among Ammon- 
oidea and their connection with the history of the impressed zone 
is of great importance in this paper. 
The duration of the habit of close coiling and involution in the 
majority of shells from the Devonian to the Cretaceous is the most 
*There is the same tendency to calcification here as in the case of the protoconch as 
compared with the membranous protoconch of Nautiloids. 
