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developed from G or B, or from C, with an intermediate transforma- 

 tion like G, or from B, with an intermediary like B", and F may give 

 rise to a similar modification when close coiled. A number of Pale- 

 ozoic forms have this outline, ex. Metacoceras. 



Diagram I. — Section of an octagonal whorl derived from H by 

 the building out of the venter and the formation of a central ven- 

 tral zone, c. v. z., two lateral ventral angles, 1. v. g., and two lat- 

 eral ventral zones, 1. v. z., ex. Tainoceras. 



Diagram J. — Section of a decagonal whorl derived from I by the 

 subdivision of the impressed zone (contact furrow) and the forma- 

 tion of a central dorsal face, c. d. f., and two lateral dorsal faces, 

 1. d. f., ex. some species of Tainoceras and Ccelonautilus. 



In their gerontic substages the whorl of G, H, I and J become 

 more or less rounded and show a tendency to return more or less 

 completely to the outline of B". 



Diagram K. — Section of an octagonal, truncated, cuneiform 

 whorl, usually derived from a whorl similar to H by the convergence 

 of the lateral zones and the subdivision of the impressed zone, 

 c. d. f., central dorsal face, 1. d. f., lateral dorsal faces. 



Diagram L. — Section of a gerontic whorl derived from K. By 

 farther degeneration the dorsal angles may disappear and the whorl 

 assume approximately the reniform outline of B. 



Diagram M. — Section of a heptagonal, cuneiform, anagerontic 

 whorl derived from K. The acute ventral angle, v. g., is formed 

 by the convergence of the lateral zones and the disappearance of 

 the abdominal angles. The dotted lines represent the similar trans- 

 formation which subsequently takes place in the same form on the 

 dorsum by the convergence of the lateral dorsal faces. The whorl 

 then becomes a hexagonal cuneiform. This outline has been repre- 

 sented with rounded umbilical shoulders, but these have to be con- 

 sidered as equivalent to two angles, and they are often more or less 

 angular. 



Diagram L. — Section of a metagerontic whorl of K or M; sim- 

 ilar forms may also result from the paragerontic degeneration of Q, 

 ex. Stroboceras sulcifer, sp. De Koninck. 



Diagram JV. — Section of a paragerontic whorl of L. The dotted 

 line represents the obliteration of the zone of impression which 

 may take place in very old whorls in this substage or in the aper- 

 tures of the ephebic stages of phyloparagerontic species. 



