﻿427 



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

 whorl with a concave abdomen in which a gibbous, central, dorsal 

 face is formed and the lateral dorsal faces are excessively narrow. 

 This may be derived from E by involution and the formation of 

 umbilical shoulders and umbilical zones, ex. Apheleceras, Subcly- 

 menia. 



Diagram P. — Section of a highly complicated fluted whorl with 

 concave abdomen. The venter has become subdivided into a fluted 

 central ventral zone, c. v. z., and two fluted lateral ventral zones, 

 1. v. z., these having become- incorporated with the lateral aspect, 

 and the intermediate lateral ventral angles, 1. v. g., form the bor- 

 ders of what is usually considered as the sides of the whorl. 



The lateral zones lying between the abdominal shoulders, a. g., 

 and the umbilical shoulders, u. s., have become subdivided into 

 two lateral faces, the outer one, 1. f. , is a broad flute, and the inner 

 one is subdivided into three lateral facets, 1. t, two of them, the 

 outer and inner facets, fluted, and one of them, the central one, 

 slightly gibbous and ridged. 



The contact furrow has a central dorsal facet, c. d. t., two lateral 

 dorsal facets, 1. d. t., and two lateral dorsal faces, 1. d. f., the angle 

 between 1. d. t. and c. d. t. is the tertiary dorsal angle, t. d. g., but 

 is not lettered, and the angle between 1. d. f. and 1. d. t. is the dor- 

 sal face angle and is also not lettered in this diagram, but these are 

 lettered in Diagram Q. The facets are introduced by the subdivis- 

 ion of the central dorsal face, which is at first flat, as in L. 



The secondary lateral angle, s. 1. g., is developed between the 

 flute of the lateral face, 1. f., and the outer facet of the inner lateral 

 face that extends from s. 1. g. to u. s. The lateral facets formed 

 out of the surface of this face are three in number, marked 1. t., 

 and the angles between these are the tertiary lateral angles, but are 

 not lettered. The angles on the central gibbous lateral facet are 

 due to longitudinal strise. 



Diagram Q. — Section of a gerontic whorl of P. The flirtings 

 and other ornaments have been obliterated, but the impressed zone 

 retains its peculiar characteristics. The more advanced parageron- 

 tic substage would approximate to Diagram N, but with more de- 

 pressed venter. Coloceras is a phyloparagerontic form, having an 

 almost reniform whorl in the neanic and ephebic stages. 



