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Gy roc eras minus calum (PL 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. 



Hercoceras (Adelfihoc) secundum, sp. Barrande, is a giant 

 form of this genus, with the impressed zone retained in the gerontic 

 stage. 



Hercoceras irregularis. 



Hercoceras mirum, var. irregularis, Barrande (PL xliii). PL 

 viii, Figs. 14-15. 



This is a distinct species having different and less closely coiled 

 young than the typical Hercoceras 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 neanic volutions have very 

 coarse, transverse ridges without any longitudinal markings. 



The caecum 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. 



