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 coste 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, Pl. iv) really represents 
this organ or its general location, is centren. This, however, is a 
