76 
NAIJTILOIBEA. 
constmctioD, consists of a great many lamellae which were applied 
successively to the exterior of the shell; and by absorbing them 
gradually from the inside the mollusc could, in spite of the dia- 
phragm, increase the length of its shell like other Nautili, 
The general appearance of Hercoceras, as Barrande remarks, 
suggests its affinities with Nautilus, but the form and peculiar 
position of the aperture are characters which distinguish it from 
all other genera. I therefore propose to restrict the genus Herco- 
ceras to species in which these singular features are observable, and 
not to include in it, as Professor Hyatt has done, other species, such 
as Gyroceras alatum, Barr., and TrocJioceras Jlexum, Barr., in which 
they are absent. 
The only species of Hercoceras known is H, mirum, Barr, (with 
its variety irregularis) ; its description, therefore, includes many of 
the characters of the genus. All the specimens representing the genus 
are found in one district, viz. between Klukovitz and Hlubocep, near 
Prague. They all belong to Etage G, bande g 3 (=Downtonian). 
The genus Hercoceras thus appears to have had a very limited range, 
both in time and space, though further examination of bande g 3 
might show that its horizontal range is more extended than it has 
yet proved to he. 
Hercoceras mirum, Barrande. 
1865. Hercoceras miinim, Barrande, Defense des Colonies, iii. p. 31. 
1867. Hercoceras mirum, Ban'ande, Syst, Sil. de la Boheiue, vol. ii. 
pt. i. p. 153, pi. xlii., pi. xliii. ff. 1-7 (var. irregularis), pi. cii., 
pi. ccxli. ffi 1-5, pi. cccclxxvii. case ii. 
1883. Hercoceras mirum, Hyatt, Genera of Fossil Cephalopods,” 
Proc. Boston Soc. Nat. Hist. vol. xxii. p. 283. 
Char. Shell forming about four volutions in adult examples. 
The whorls only slightly embracing. Transverse section an elon- 
gated ellipse, one side of which is emarginate. In certain specimens 
the elliptical outline becomes almost quadrate, but the ventro-dorsal 
is always less than the transverse diameter, except just at the apex 
of the shell. The bod}*-chamber occupies about half of the last 
whorl, and its manner of development corresponds with that of the 
internal volutions. In the majority of specimens it is in contact 
with the preceding whorl, but in some individuals it frees itself and 
is produced in a curved, or an approximately straight line. Such 
specimens are classed by Barrande as a variety, under the name 
irregularis. The structure of the aperture and the ornamenta- 
tion of the shell have already been described under the generic 
diagnosis. 
