﻿596 



in the paranepionic because of mechanical and not through genetic 

 causes. 



In the Trocholitidae the straight and arcuate forms are not yet 

 known nor are the nautilian forms quite satisfactory. 



The dorsal side of Litoceras insolens, which has a comparatively 

 large umbilical perforation, has not yet been studied in the parane- 

 pionic substage, and although it seems very likely that it is gibbous 

 and without a dorsal furrow, this cannot be stated positively. In 

 Trocholitoceras and Trocholites, the umbilical perforations are very 

 small and have dorsal furrows on the paranepionic after the gyroce- 

 ran bend has been passed by. 



The same argument can be framed for their appearance that was 

 used for the Tarphyceratidae, viz., that the weight of evidence is 

 in favor of the mechanical generation of the dorsal furrow in the 

 paranepionic. There is also one fact possibly of some importance 

 in this connection. In the specimen of Trocholites canadensis, 

 in section Figs. 39 and 40, PI. vi, it can be seen that the inner part 

 of the dorsal furrow, where it first appears, is a single, broad fur- 

 row. As it becomes more distant from the gyroceran bend, how- 

 ever, it becomes divided into two smaller furrows by the rising of 

 a central gibbous face. 



It might be assumed that the development of this central gibbous 

 face was due to heredity, this being the expression of a tendency to 

 return to the rounded dorsum of radical types as soon as the pres- 

 sure due to the abrupt bending was removed. 



Precisely similar furrows and a median gibbous face occur, how- 

 ever, on the dorsum of Cranoceras. The curvature of this form and all 

 of its characteristics indicate that the bending of the cone could not 

 have been the mechanical agent which caused a single dorsal furrow 

 and the appearance of the two dorsal furrows and the central gib- 

 bous face complicates the problem and seems to make it insoluble 

 on a purely mechanical basis. I have called this a gibbous face, 

 but in reality it is not a "face " at all in the sense in which that 

 term is here used. It is a modification of the primitive rounded 

 dorsum and is really a "zone" or secondary modification. In 

 Trocholites it arises as a modification of the dorsal furrow, and is 

 therefore a true " face." It is possible that with advance of knowl- 

 edge this distinction may be more important than it seems now, and 

 may enable us to explain the exceptional characteristics of Cranoceras. 



Before it could be safely assumed that mechanical causes gener- 



