﻿425 



of the whorl,* and may be an intermediate stage leading into a 

 whorl like that shown in H, or it may acquire lateral angles as in 

 C, thus passing into G, or A may pass directly into H. 



Diagram C. — Section of a digonal whorl with primitive regions 

 and lateral angles, 1. g., occurring in the ephebic stage of ortho- 

 ceran and cyrtoceran forms and in the young of nautilian forms, 

 Edaphoceras. 



Diagram D. — Section of a trigonal whorl with gibbous venter, 

 lateral angles, 1. g., and projecting dorsal angle, p. d. g., ex. Tri- 

 gonoceras. 



Diagram E. — Section of a trigonal, shield-shaped whorl, with 

 concave venter, lateral angles, 1. g., and projecting dorsal angles, 

 p. d. g. Either D or E may evolve into a tetragonal whorl by the 

 appearance of a lateral zone on the outer part of the sides and the 

 rounding off and disappearance of the dorsal angle, ex. Trigono- 

 ceras. 



Diagram F. — Section of a tetragonal whorl with gibbous venter 

 and dorsum and lateral zones, 1. z. This may be developed from 

 B" or from C. 



The morphic distribution of these forms is as follows : A, B and 

 C may be Orthoceran, Cyrtoceran or Gyroceran, but are more gen- 

 erally Orthoceran ; D may be Orthoceran, but is usually Cyrto- 

 ceran and Gyroceran ; E and F are almost exclusively Gyroceran. 

 All of the remaining outlines belong to Nautilian forms. 



Diagram G. — Section of a tetragonal, trapezoidal whorl with a 

 contact furrow nearly as broad as the dorsum, the sides flat and 

 well defined. This may be evolved from C or B" in development 

 of Nautilian forms. The abdominal angle, a. g., in this form is 

 derived from the lateral angle of forms like C. Sides, s., are still 

 undivided, ex. Temnocheilus. 



Diagram H. — Section of a hexagonal whorl with lateral zones, 

 1. z., developed between the abdominal shoulders or angles, a. g., 

 and the umbilical shoulders, u. s., and umbilical zones, u. z., devel- 

 oped between the latter and the lines of involution, 1. in. The 

 contact furrow remains primitive or undivided. This may be 



* This same diagram can also be used to represent the paragerontic substage of the de- 

 generation. A reniform whorl may result in the gerontic stage from such an ephebic 

 ■whorl as is represented in H, J, K, or P. Q shows an intermediate stage between 1* and a 

 reniform paragerontic whorl. No confusion need result from this double use of the same 

 outline, since it does not imply identity of structure, but simply the identity of form at 

 the extremes of the ontocycle in the individual and of the phylocycle in the group. 



