600 



MOLLUSCA 



PHYLUM VI 



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

 often appear at the intersections of the longitudinal and transverse hands of growth. 



Siphuncle toith faintly nummuloidal, fusiform or tuhular 

 segments. 



Kionoceras Hyatt. Longitudinal ridges present as a rule 

 only in tlie earlier stages, after wliich inconspicuous annuli 

 appear, but with some few exceptions become obsolete before 

 the epliebic 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. 1113). Like the 

 last, but with more or less sinuous longitudinal ridges. Silu- 

 rian to Carboniferous. 



II. Plectoceratida Hyatt. 



Orthoceracones, gyroceracones, and very discoidal naittilicones 

 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 longitudinal striae or 

 Thoracomms r'u-hniaium fi''^'^ ridges, especially in the young, btht these generally disappear 

 (Barr.). Silurian (Etage before the ephehic stage. Siphuncular segments slightly num- 



15): Dvoretz, Bohemia (after 7 • j 7 -f -^ j 7 7 



Barrande). m.'ulmdal, jusijorm or tubular. 



Family 4. Tarphyceratidae Hyatt. 



Orthoceracones, cyrtoceracones, gyroceracones and nautilicones, compressed oval in 

 section, venter narroioer than the dorsum. Shell smooth or sometimes ivith primitive fohl- 

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



Ap)hctoceras, Deltoceras, Barrandeoceras, Tarphyceras Hyatt ; Planctoceras, Eury- 

 stomites Schroder ; Falcilituites Eemele. Ordovician. (For descriptions see Hyatt's 

 Phytogeny, 1894.) Eurystomites and Tarphyceras are wholly nautilicones, the remain- 

 ing genera are eitlier cyrtoceracones or gyroceracones. 



Family 5. Trocholitidae Hyatt 



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

 from them in the young. As a ride 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 Eemele). Ordovician. JDiscoceras Barrande. Ordovician 

 and Silurian. 



Family 6. Plectoceratidae Hyatt. 



Gyroceracones, nautilicones and torticones having annular costae from the neanic 

 stage until late in life, and in some genera, more or less prominent longitudinal ridges, 

 which usually disappear in the ephebic stage. Siphuncle ventrad of centre. 



