520 



MOLLtfSCA 



SUB-KINGDOM VI 



Family 6. Plectoceratidae. 



Gyroceracones, nautilicones, and torticones having annular costae from the neanic 



stage until late in life, and in 

 some genera, more or less pro- 

 minent longitudinal ridges, 

 which usually disappear in 

 the ephebic stage. Siphuncle 

 ventrad of centre. 



Plectoceras, Hyatt, Ordo- 

 vician and Silurian. Sphy- 

 radoceras, Hyatt (Peismoceras, 

 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 troclioceran type. 



Family 7. Ophidio- 

 ceratidae. 



FIG. 1065. 



Sphyradoceras optatum, Barr. sp. Silurian 

 (Etage E) ; Lochkow, Bohemia (after Barrande). 



Discoidal nautilicones, 

 costated from the neanic stage 

 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 crenulated ridges ; the dorsum 

 gibbous and rounded. Siphuncle near 

 the venter. Devonian. 



FIG. 1066. 



Ophidioceras simplex, Barr. 

 Silurian (E) ; Lochkow, Bohemia. 

 Vi (after Barrande). 



Family 8. Lituitidae. 



Excepting the supposed ancestral, primitive genus, Cyclolituites, this 

 is a series of phylogerontic uncoiled forms with an extreme modification 

 in the almost completely uncoiled Rhynchorthoceras. Apertures quite 



distinct from those of the preceding family ; hyponomic sinus shallower, Montf. Ordovician 

 ., j ii i * i 7 drift ; East Prussia. 



there are narrow ventro-lateral crests, and small lateral sinuses and i/ ( a rter Noetiing). 



FIG. 1067. 

 Lituites lituut 



