96 



Report of the State Geologist. 



In the course of the early volutions the septa are relatively more distant 

 than in the later growth of the shell. This appears to be a regular increase 

 in degree of septation. Thus there are seven or eight septa in the first volu- 

 tion, eleven in the second, fifteen or sixteen in the third and as many as forty 

 or forty-five in the fifth. 



Distinction of the Stages of Growth. It has already been remarked that the 

 growth-stages, which are pretty clearly defined in the species Mcmtic. Patterson i 

 are here somewhat obscured on account of the absence of sharply marked sur- 

 face sculpture. Nevertheless by close analysis of this 

 feebly developed trait aided by the evidence afforded 

 by variations in septation, umbilication and whorl section, 

 it becomes possible to deduce some rational inferences 

 regarding the division and duration of the growth 

 periods. 



The nepionic period in some other species is repre- 

 sented, as we have noticed, by a smooth shell, which 

 in Mantic. Pattersoni, extends for about a half- volution. 

 In ProbeJ. Luther i this shell is also smooth and 

 the best of our preparations indicate that the smooth- 

 ness of surface was continued for at least one entire 

 volution. It is also evident that an ananepionic substage is 

 defined by the retention of the embryonic convexity of the 

 shell over the incipient part of this conch, a metanepionic 

 substage by the abrupt decrease of diameter of the whorl 

 section, and a paranepionic condition by the gradual 

 return to a normal rate of whorl expansion. If the 

 nepionic stage of growth was longer continued than in 

 some other species, as suggested by the smoothness of 

 the entire first whorl, its duration was an increase only in 

 the paranepionic substage, for the embryonic, ananepionic 

 and metanepionic conditions correspond throughout in 

 the amount of shell formed, with the same stages in 

 Mitotic Pattersoni. The termination of the smooth 



Figure 77. Probe I o c era s 



Lutheri. vertical section from surface affords a whorl section which is notable as 



protoconch to adult whorl 



showing successive loss and show ing the first inclination to Lateral flattening. So 



gain of umbilication, the vari- 

 ation in the form of the whorl far asour evidence now extends, there is, in the septation 



section and the modificationsof _ m 



the ventral sulcus. Enlarged. () f the shell, no innovation w 1 1 it'll comes in w ith the 

 assumption of the surface ornament and lateral compression of the whorl. 

 Throughout this period the septum lias probably maintained its form without 



