﻿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 Trocholitoceras Walcotti, Fig. 

 12, PI. 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. 



