493 
Gyroceras minusculum (P|. xxx) may also prove to be related to 
species of this genus. 
The young of these forms are all closely coiled and have con- 
tact furrows when the volutions are in contact, and these are 
also retained more or less in the gerontic stages. 
The section is at first rounded, then broadens out to a digonal 
form, which in some species may remain more or less digonal or 
become quadragonal. 
The young of Hercoceras has the subquadragonal form in some 
species like that of the adults of Trochoceras. 
fercoceras (Adelphoc) secundum, sp. Barrande, is a_ giant 
form of this genus, with the impressed zone retained in the gerontic 
stage. 
HERCOCERAS IRREGULARIS. 
HERCOCERAS MIRUM, vay. IRREGULARIS, Barrande (Pl. xliii). Pl. 
vill, Figs. 14-15. 
This is a distinct species having different and less closely coiled 
young than the typical Mercoceras mirum, and is transitional be- 
tween Hercoceras and Ptenoceras. The nepionic stage given in 
Figs. 14-15, Pl. viii, shows that the metanepionic whorl is a de- 
pressed ellipse, the paranepionic volution is more rounded, the 
ventro-dorsal slightly longer than the transverse diameter, and the 
neanic whorl may be subquadragonal, or pass from this directly 
to the digonal form of the ephebic stage. 
The exterior of the nepionic and néanic volutions have very 
coarse, transverse ridges without any longitudinal markings. 
The czecum is large in the apex. It is not correctly given in 
Fig. 15, and is ventrocentren. In another specimen at a some- 
what younger age, it was very large compared with the diameter of 
the ananepionic volution and centren. In this also it remained in 
the mesal plane in later ages, although shifting to propioextraven- 
tran position. 
The umbilical perforation is small and comma-shaped, and 
although it seems impracticable that the paranepionic whorl should 
succeed in growing around the apex without enveloping it, this 
really occurs, and no impressed zone is formed in the ananeanic 
substage. The volutions come in contact later, and a faint con- 
tact furrow appears in the metaneanic substage, which becomes 
deeper in the ephebic stage as figured by Barrande. 
