471 
rapidly until it reaches the centrodorsan position in the anephebic 
substage at the beginning of the third whorl. 
The ventro-dorsal diameters also slowly decrease by growth cor- 
relatively with this movement, along the mesal plane and proceed 
with equal steps, correlative with changes in the septa, and relative 
dimensions and shapes of the air chambers and the shifting of the 
siphuncle towards the dorsum to the first quarter of the third whorl 
where they take on the adult proportions and aspect. 
These facts are admirably well shown in the figures of Schroedero- 
ceras (Lit.) teres by Holm, reproduced here if allowance is made 
for the more cyrtoceran or less involute form of Ha/sont, which 
has a larger umbilical perforation. The third septum in both 
forms, however, comes internally to the same point, the end of the 
cyrtoceran stage, when the whorl makes a sudden bend and assumes 
the gyroceran curvature that brings it at the end of the first whorl 
against apex of the conch. In Figs. 21 and 22, from Holm, this 
bend is more abrupt and more like that of Trocholites than in this 
species. The dorsal side of the last quarter of the first whorl 
actually strikes and lies upon the dorsal side of the first air cham- 
ber, whereas in this species the contact takes place farther towards 
the apex. In ¢eves also, according to Holm’s figures, the approxi- 
mation of the siphuncle towards the dorsum takes place more 
rapidly and probably earlier than in Ha¢onz. Holm found no signs 
of a cicatrix on the apex of feres, but no shell is represented in his 
figures and he describes the whorls as so very closely approximated 
that there was but one shell wall. The young shell is very thin, 
and probably this explains the difficulty of separating the whorls. 
At any rate, the absence of the cicatrix is not established by his 
observations. I think he must have overlooked the shell wall, this 
not being absent in any other forms that I have examined. 
Fig. 34, Pl. vi, gives the aspect of an accidental section, the 
location of which is shown by the line through Fig. 35, taken from 
the center of Whitfield’s original of this species. The sections 
passed subdorsan to the shell, cutting across the two first septa of 
the metanepionic substage. The peculiar aspect of this part of the 
section is due to the continuity of the lateral shell lines on either 
side with those of the paranepionic whorl which is given in section 
of volution immediately under this. The convex line dividing the 
metanepionic from the paranepionic volution, the projecting third 
septum. The reverse, the splinter from which this section was 
