CLAss V CEPHALOPOJ)A 599 



Protobactrites Hyatt. Long i)encil-shaped orthoceracones and cyrtoceracones, 



Fio. 1100. 



Orthocerasinter medium Marklin. Siluriaii ; 

 Gottland. Longitudinal section showing 

 siplmncle, septa and pseudosepta ; camerae 

 lilled up witli calcite. 



Fig. 1110. 



Orihoceras michelini 

 Barr. Silurian ; Kozorz, 

 Boheniia. Longitudinal 

 section showing sliort 

 sii)honal funnels. 



circiilar, or compressed elliptical in section, ornamented with 

 transverse and sometiines longitudinal Striae. Siphuncle 

 tubulär, centren or near the centre. Truncation occurs in soiiie 

 species, and others are more or less transitional to Bactrites 

 among the Ammonoids. Type P. {Oriho- 

 ceras) styloideum (Barr.). Silurian to Car- 

 boniferous. 



Family 2. Oycloceratidae Hyatt. 



Fio, 1111. 



Geisonoccras timidum 

 (Barr.). Silurian ; Loch- 

 kow, Bohemia. 



Orthoceracones and cyrtoceracones having annuli with transverse 

 Striae or bands of growth at all stages ; longitudinal ridges^ when 

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

 large siphuncles, and are apparently more directly connected with 

 primitive Endoceratida than with the Orthoceratidae. 



Protocycloceras Hyatt. Annulated orthoceracones and cyrto- 

 ceracones without longitudinal ridges. Siphuncle large. Type 

 P. {Orihoceras) lamarcki (Bill.). Ordovician. 



Cycloceras M'Coy {Dictyoceras, Heloceras Eichw.). Annulated 

 orthoceracones and cyrtoceracones with discontinuous longitudinal 

 ridges. Siphuncle generally tubulär or with fusiform segments ; 

 deposits when present irregulär as in Orihoceras. Annuli often 

 become obsolete in paragerontic stages. Ordovician to Perniian. 



Daiüsonoceras Hyatt (Fig. 1112). Siniilar to Cycloceras, but 

 having prominent frilled bands of growth between and on the 

 annulations, the frills sometimes forming more or less discon- 

 tinuous longitudinal ridges. Silurian and Devonian. 



Gtenoceras Noetling. Cyrtoceracones like Dawsonoceras dulce 

 (Barrande), but with fine longitudinal ridges between the annuli, 

 and living chamber with three internal folds or processes — one 

 median dorsan, and a pair on the venter. SiiJhuncle dorsad of 

 centre. Ordovician. 



Family 3. Kionoceratidae Hyatt. 

 Orthoceracones and cyrtoceracones with more or less well-marked continuous longi- 



FiG. 1112. 



Dawsonoceras annula- 

 tum (Sowb.). Silurian 

 (Etage E) ; Viscocilka, 

 Bohemia. Terminalpor- 

 tion showing Shell of 

 living Chamber and 

 sectioned camerae (after 

 Barrande). 



