518 MOLLUSCA SUB-KINGDOM vi 



Orthoceras, Breyn (Figs. 1060, 1061). Long tapering orthoceracones and cyrto- 



FIG. loco. 



Orthoceras intermedium, Marklin. Silurian ; 

 Gottland. Longitudinal section showing 

 siphuncle, septa, and pseudosepta ; camerae 

 filled up with calcite. 



FIG. 1061. 



Orthoceras Michelini, 

 Barr. Silurian ; Kozorz, 

 Bohemia. Longitudinal 

 section showing short 

 siphonal funnels. 



FIG. 1062. 



Geisonoceras timid-urn, 

 Barr. sp. Silurian ; Loch- 

 kow, Bohemia. 



ceracones, smooth, or with only transverse striae and growth 



bands. Siphuncle generally larger than in Geisonoceras, centren 



or slightly dorsad of centre. Deposits when present gathered 



about the funnels as in Annulosiphonata. Silurian to Trias. 



Geisonoceras, Hyatt (Fig. 1062). Similar 

 to the last, but sides spreading more rapidly, 

 and siphuncle empty, centren, or slightly 

 ventrad of centre. Ordovician to Carbon- 

 iferous. 



Protobactrites, gen. nov. Long pencil- 

 shaped orthoceracones and cyrtoceracones, circular or compressed 

 elliptical in section, ornamented with transverse and sometimes 

 longitudinal striae. Siphuncle tubular, centren or near the centre. 

 Truncation occurs in some species, and others are more or less 

 transitional to Bactrites among the Ammoiioids. Type P. (Orth.~) 

 styloideum, Barr. sp. Silurian to Carboniferous. 



Family 2. Cycloceratidae. 



Orthoceracones and cyrtoceracones having annuli with 

 striae or bands of growth at all stages ; longitudinal ridges, when 

 present, more or less discontinuous. The earliest forms often have 

 large siphuncles, and are apparently more directly connected with 

 primitive Endoceratida than with Orthoceratidae. 



Protocycloceras, gen. nov. Annulated orthoceracones and cyrto- 

 ceracones without longitudinal ridges. Siphuncle large. Type P. 

 (Orth.) Lamarcki, Bill. sp. Ordovician. 



Cycloceras, M'Coy (Dictyoceras, Heloceras, Eichw.). Annu- 

 lated orthoceracones and cyrtoceracones with discontinuous longi- 

 tudinal ridges. Siphuncle generally tubular or with fusiform 

 segments; deposits when present irregular as in Orthoceras. 

 Annuli often become obsolete in paragerontic stages. Ordovician 

 to Permian. 



Da wsonoceras, Hyatt (Fig. 1063). Similar to Cycloceras, but 

 frilled bands of growth between and on the annulations, the frills 



FIG. 1003. 



Davsonoceras annida- 

 tum, Sowb. sp. Silurian 

 (E) ; Viscocilka, Bo- 

 hemia. Terminal por- 

 tion showing shell of 

 living chamber and 

 sectioned camerae (after 

 Barrande). 



having prominent 



