409 
shells which do not have an impressed zone at any stage in Bar- 
randeoceras while the dorsum is still convex, and in Waut- 
lus aratus it and the annular lobe is found beginning in the third 
septum, and similar observations have been made on a few other 
species in the descriptive part of this memoir. The characteris- 
tics of the ananeanic substage of JV. pompilus show how distinct 
this substage is in existing nautilus from the preceding and suc- 
ceeding substages. The longitudinal ridges disappear during this 
substage, and the broad transverse bands of growth become in con- 
sequence for a time more prominent. ‘The uniform brown of the 
paranepionic may begin to be striped on the sides in the latter part 
of the same substage, but this is often delayed until the ananeanic 
substage and always become more definite at this time. 
In the metaneanic substage the shell becomes smooth, the brown 
striping extends on to the venter, and the markings become more 
distinct and more widely separated. The whorl which, during the 
preceding substage, had lost the subtrigonal outline of the para- 
nepionic and become kidney-shaped in outline, with a deep im- 
pressed zone, now acquires a deeper impressed zone and slightly 
flattened sides and venter, thus forming lateral zones, as in Vautilus 
umbilicatus, and repeating at this stage the form of whorl character- 
istics of that species. During the paraneanic substage the deposits 
of porcellanus matter in the umbilical zone begin but do not 
become a very marked characteristic. 
In the ephebic stage these deposits on either side increase and 
the whorl spreads inwardly closing the umbilici, the whorl in the 
meantime losing its flattened venter, which again becomes rounded. 
The metephebic substage begins when the umbilical perforations 
become obliterated by the ingrowth of the umbilical zones. 
The parephebic substage is definable externally only by the ces- 
sation of the coloration. ‘This may be due either to the fact that 
senility is not marked by any peculiar structural changes, as hap- 
pens often-in other highly involute species of Nautiloids and even 
in many Ammonoids with smooth shells, or because no very large 
old specimens have been collected. 
These remarks do not represent fairly all the ontogenic changes 
in existing Nautili, which will be treated in another essay, but they 
suffice for the purposes of this paper and serve, with other facts 
cited, to show the applications of the nomenclature used in the 
following pages. 
PROC. AMER. PHILOS. SOC. XXXII. 148. 22. PRINTED JUNE 6, 1894. 
