SUB-ORDKK I) 



OKTHOCHOANITKS 



:, 1 1 



sometimes forming more or less discontinuous longitudinal rid^->. Silurian and 

 Devonian. 



Ctenoceras, Noetl. Cyrtoceracom-s likr / > n-sonoceras </"/<<, I'.an. .-]., hut with 

 fine longitudinal ridg<- Mweun tin- annuli, and living chamber with ilm-,- intermfl 

 folds or processes one median dorsan, and a paii -on ih.- v.-nti-r. Siph uncle dorsad 

 of centre. Ordovician. 



Family :?. Kionoceratidae. 



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

 tudinal ridges, and either with or without annulations. Spinous processes or tubercles 

 often appear at the intersections of the longitudinal and transverse 

 bands of growth. Siphuncle with faintly nummuloidal, fusiform, 

 or tubular segments. 



Kionoceras, Hyatt. Longitudinal ridges present as a rule 

 only in the earlier stages, after which inconspicuous annuli 

 appear, hut with some few exceptions hecome obsolete before the 

 ephebic stage. Silurian to Carboniferous. 



Spyroceras, Hyatt. Very long, slender, annulated shells, 

 with more or less prominent longitudinal ridges in the ephebic 

 stage. Ordovician to Carboniferous. 



Thoracoceras, Eichw. (Melia, Eichw.), (Fig. 1064). Like the 

 last, but with more or less spinous longitudinal ridges. Silurian 

 to Carboniferous. 



II. PLECTOCERATIDA. 



Orthoceracones, gyroceracones, and very discoidal nautilicones 

 with comparatively slight impressed zone. Volutions of gerontic 

 stage often have a centrifugal tendency, becoming sometimes 

 straight and even bending slightly in the opposite or ventral 

 direction. Shells annulated or costated, and often with longi- Flo 10( . 4 



tudinal striae or fine ridges, especially in the young, but these Thomcocem .< corbuintum, 

 generally disappear before the ephebic staqe. Siphunciilar sea- Barr. sp. Silurian (Stage 

 ,.,,- E);Dvoretz. Bohemia (after 



ments slightly nummuloidal, fusiform, or tubular. Harrande). 



Family 4. Tarphyceratidae. 



Orthoceracones, cyrtoceracones, gyroceracones, and nautilicones, wm$rf*fd <>rn} /// 

 section, venter narrower than the dorsum. Shell smooth or sometimes witli primitive fold- 

 like costae. Siphuncle empty, tubular and ventrad of centre. 



Aphetoceras, Deltoceras, Barrandeoceras, Tarphyceras, Hyatt ; Planctoceras, Eury- 

 stomites, Schroder ; Falcilituites, Remele. Ordovician. (For descriptioiLS see Hyatt's 

 Phylogeny, 1894.) Eurystomites and Tarphyceras are wholly nautilicoiu-s, the remain- 

 ing genera either cyrtoceracones or gyroceracones. Orthoceracones represented by 

 genera at present undescribed. 



Family 5. Trocholitidae. 



Nautilicones resembling those of the preceding family, and not easily distinguished 

 from them in the young. As a rule they have excessively broad volutions with reniform 

 section and an impressed zone at a very early age; the siphuncle is then ventrad of the 

 centre, but in the ephebic stage it is tubular and dorsad of centre. 



Schroederoceras, Litoceras, Trocholitoceras, Hyatt ; Trocholites, Conrad (Palaeo- 

 nautilus, Palaeoclymenia, Remele). Ordovician. Discoceras, Barrande. Ordovician 

 and Silurian. 



