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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, i. 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 Nautilinidse are the only 

 representatives of the Goniatitinse 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 Nautilinidse, 

 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 Goniatitinae 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 

 Nautilinidse 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 is 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 confirmation. 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. 



