56 NAUTILIDA. 
HeErcoceras, Barr., 1865. 
Htym.—Erkos, a wall, ceras, a horn. 
Distr.—2 sp. Middle Silurian, Bohemia; Devonian, Nassau (7). 
Shell generally nautiloid, the whorls sometimes separated, or 
even turbinate ; body-chamber with a diaphragm perpendicular 
to the axis of the shell, the concavity of which is opposed to 
that of the last septum, throwing the aperture on the deeply 
excavated dorsal side of the shell; siphuncle ventral, cylindrical, 
inflated between the chambers, separated from the shell. 
LitvitEs, Breyn., 1732. 
Etym.— Lituus, a trumpet. 
Syn.—Trocholites, Emmons, 1842. Paleonautilus and Paleo- 
clymenia, Remelé. 
Disir.—28 sp. Silurian; Europe, North America. JZ. sim- 
plex, Barr. (sec30)): 
Shell planorbiform, the whorls close or separate; the last 
chamber produced in a straight or outwardly curved line; lat- 
eral margins of the aperture extended and curved towards the 
interior of the shell, contracting the aperture into two distinct 
orifices. 
OPHIDIOCERAS, Barrande, 1867. Htym.— Ophiodes, serpent- 
shaped, ceras, a horn. Shell with the produced portion very 
short or wanting. Distr.—7 sp. Silurian; Norway, Bohemia. 
STROMBOLITUITES, Remelé, 1881. Shell commencing as a small 
spiral, expanding into an obconic form. L. Silurian; Germany. 
S. Torelli, Remelé (xxix, 90). 
HorTOLUS, Montf., 1808. Whorls not in contact. 
DiscoceraAs, Barrande, 1867. 
Etym.—Diskos, a quoit, ceras, a horn. 
Distr. sp. Middle Silurian ; Russia, Germany, Norway. 
Shell planorbiform ; produced portion very short or wanting ; 
aperture simple, not contracted. 
M. Barrande describes this as a subgenus under his genus 
Lituunculus; of which no species have been observed, but which 
he creates by anticipation with che diagnosis: ‘ Shell like Litu- 
ites, but with a simple aperture,” in order that Discoceras may 
hold the same relationship to it that Ophidioceras does to Litu- 
ites! This is filling up the ‘ gaps” with a vengeance, and could 
scarcely have been predicted of the renowned Bohemian anti- 
developmentalist. 
PTERONAUTILUS, Meek, 1867. 
Htym.—FPteron, a wing, and Nautilus. 
Distr.—P. Seebachianus, Geinitz (xxxi, 10). Permian. 
Shell spiral, involute, finally produced, with lateral wing-like 
expansions. 
