﻿440 



to be, as stated above, a bow-shaped, dark, smooth filling, as shown 

 in Fig.' 5 and more enlarged in Figs. 6 and 7. Fig. 8 is an ideal 

 restoration of a side view of the nepionic stage, and gives the loca- 

 tion of sections shown in Figs. 6 and 7. By the aid of Fig. 7 one 

 can see that the furrow which appears in the dorsum of the 

 paranepionic substage is first found just as the whorl makes the 

 sharp turn to form the umbilical perforation. This shows also that 

 its origin may be purely mechanical. The hard wall of the dorsum 

 of the metanepionic was only about .5 mm. distant from the grow- 

 ing pliable edge of the paranepionic as it made the turn, and this 

 pliable border may have been built to conform to the shape of the 

 internal metanepionic dorsum. This becomes possible when one 

 takes into consideration the rapid growth of the whorl in its lateral 

 and ventro-dorsal diameters at this stage. The increase of the 

 former broadening out the volution causes the involution of the 

 apex on the sides when this is reached, and rapid increase of the 

 ventro-dorsal diameters forces the building shell to make this sud- 

 den turn, owing to the more rapid building out of the ventral side. 



Immediately after passing this point of greatest pressure, as shown 

 in Fig. 6, the zone produced by it begins to decrease in depth and 

 increase in width, but it does not disappear altogether, because the 

 growing shell immediately strikes the dorsal side of the metane- 

 pionic and ananepionic substages and the true contact furrow 

 appears. This is shown in the truncation of the dorsal corner of 

 the outline in Fig. 8 when it strikes the apex. The centre of Fig. 5 

 is approximately the same as Fig. 6. 



A Trocholites-like outline is assumed in the neanic stage (shown 

 in Fig. 5 in section of second whorl below center) and in the ephe- 

 bic stage the whorl is apt to become slightly flattened on the venter. 

 The outer whorl of section, Fig. 5, is flattened in this way and 

 represents the anephebic condition of the living chamber. 



This shell is smooth until the ananeanic substage, as in Fig. 4, 

 and then becomes costated. These costae are infrequent, low, 

 broad elevations which become less distinct with the incoming of 

 the anephebic substage and are very often absent in the later ephe- 

 bic substages, beginning however again in the gerontic stage, but 

 are never so constant or prominent as in the earlier stages. 



The siphuncle of the metanepionic whorl, if the mark in the 

 centre of the enlarged outline (Fig. 11, PI. iv) really represents 

 this organ or its general location, is centren. This, however, is a 



