MOLLUSCA 
n 
bo 
© 
Family 6. Plectoceratidae. 
SUB-KINGDOM VI 
Gyroceracones, nautilicones, and torticones having annular costae from the neanie 
stage until late in life, and im 
some genera, more or less pro- 
minent longitudinal ridges, 
which usually disappear wn 
the ephebic stage. Siphunele 
ventrad of centre. 
Plectoceras, Hyatt. Ordo- 
vician and Silurian. Sphy- 
radoceras, Hyatt (Persmoceras, 
Systrophoceras, Hyatt), (Fig. 
1065). Silurian and De- 
vonian. The first is gyrocera- 
conic, with some discoidal 
nautilicones, and the second 
is almost exclusively torticonic 
of the trochoceran type. 
Family 7. Ophidio- 
ceratidae. 

Discoidal nautilicones, 
costated from the neanic stage 
Fic. 1065. 
Sphyradoceras optatum, Barr. sp. Silurian : Ta “ 
(Etage E); Lochkow, Bohemia (after Barrande). onward. Volutions of the 
young small and numerous. 
Section during ephebic stage generally compressed, venter narrower than 
the dorsum. Siphuncle tubular, small. 
Ophidioceras, Barr. (Fig. 1066). Nau- 
tilicones with straight lateral costae and 
raised bands on the venter, and longitudinal 
ridges in the young. Siphuncle dorsad or 
ventrad of centre during ephebic stage, but 
ventrad during nepionic. Gerontic aper- 
tures with prominent dorsal and lateral 
crests, and very deep hyponomic sinus. 
Silurian. 
Homaloceras, Whiteaves. Cyrtocera- 
cones with section similar to that of Ophid- 
ioceras, venter narrow and channeled, 
bordered by ecrenulated ridges ; the dorsum 
mS gibbous and rounded. Siphuncle near 
Fic, 1066, the venter. Devonian. 
Ophidioceras simples, Barr. 
Silurian (BE); Lochkow, Bohemia. 
1 ‘ % ~ , e eye 
i Ober eatranae). Family 8. Lituitidae. 

Excepting the supposed ancestral, primitive genus, Cycloliturtes, this 
is w series of phylogerontic uncotled forms with an extreme modification 
in the almost completely uncoiled Rhynchorthoceras. Apertwres quite 
distinct from those of the preceding family ; hyponomic sinus shallower, 
there are narrow ventro-lateral crests, and small lateral sinuses and 


Fic. 1067. 
Lituites lituus, 
Montf. Ordovician 
drift ; East Prussia. 
1/y (after Noetling). 
