478 
distinct from those of full-grown shells. ‘The siphuncle was propio- 
dorsan, being a shade less than its own diameter removed from 
that surface, its diameter being 3 mm. 
The whorl is still kidney shaped in section in this substage, with 
rounded lateral zones, elevated rounded abdomen and rounded 
abdominal angles, but there is an evident tendency to broaden on 
the sides and to form steep, horizontal umbilical zones. These 
parts, being developed out of a digonal whorl, have the usual primi- 
tive form of the kidney-shaped whorl, but the slightest flattening 
of the lateral zones would convert the section into a quadragonal 
outline. There is a deep wide contact furrow at this age, and the 
involution completely covers the abdomen of the next inner whorl 
to the abdominal angles, the umbilical zones actually bulging 
inwards, and encroaching somewhat on the umbilici, comparing 
closely with the younger stages of Zrocholitoceras Walcott, Fig. 
mes Ply vi. 
The living chamber of this specimen, Fig. 9, although incom- 
plete, was nearly one-half of a volution in length at the end of the 
third volution. About one-quarter of a volution from the septal 
floor it measured in transverse diameter 33 mm. and the ventro- 
dorsal 15 mm., showing that the rate of growth in the transverse 
diameter had begun to lessen considerably. The depth of the 
umbilicus measured from the umbilical shoulders was 15 mm. at 
about the end of the last half of the third whorl. The shell was 
very thick on the venter even at this age. 
During subsequent growth the sides are apt to become broader and 
flatter, but the transverse diameters always exceed considerably the 
ventro-dorsal. The fossils are all apt to be more or less distorted 
by pressure, so that it is difficult to draw the line between this species 
and Litoceras Whiteavesi, except in the young. In these the whorl 
is of greater breadth and the siphuncle nearer to the dorsum than 
the last mentioned. 
As the sides become better defined the sutures change. ‘The 
ventral saddle disappears in a broad lobe or almost straight suture, 
slight saddles appear at the rounded abdominal angles and the 
broad lateral lobes ascending to the lines of involution and the blunt 
saddles, as in the young, on the primitive and rounded umbilical 
zones are all better defined. The broad deep dorsal lobe also re- 
mains as in the young in the contact furrow. 
The whorl was not free at the aperture in any specimens observed. 
