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shells which do not have an impressed zone at any stage in Bar- 

 randeoceras while the dorsum is still convex, and in Nauti- 

 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 N. pompilius 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 Nautilus 

 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. 



