600 



MOLLUSCA 



PHYLUM VI 



tudinal ridges, and either with or ivithout annulations. Spinous ])rocesses or tuhercles 

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

 Siphuncle with faintly iiummuloidal, fusiform or tubulär 

 Segments. 



Kionoceras Hyatt. Longitudinal ridges present as a rule 

 only in the earlier stages, after wliich inconspicuous annuli 

 appear, but with some few exceptions become 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. 1113). Like the 

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

 rian to Carboniferous. 



IL Plectoceratida Hyatt. 



Orthoceracones, gyroceracones, and very discoidal nantilicones 



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. 



p ^. Shells annulated or costated, and often ivith longitudinal Striae or 



Thoracoceras corbulatum -^^^ ridges, especially in the young, but these generally disappear 



(Barr.). Silurian (Etage before the ephebic stage. Siphuncular Segments slightly num- 



E); Dvoretz.Bohemia (after i -j i x -x ± i i 



Barrande). muloidal, fusiform or tubulär. 



Family 4. Tarphyceratidae Hyatt. 



Orthoceracones, cyrtoceracones, gyroceracones and nautilicones, compressed oval in 

 section, venter narrower than the dorsum. Shell smooth or Sometimes with primitive fold- 

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



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

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

 Phylogeny, 1894.) Eurystomites and Tarphyceras are whoUy nautilicones, the remain- 

 ing genera are either 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 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 tubulär and dorsad of centre. 



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

 nautilus, Palaeoclymenia Remele). Ordovician. Discoceras 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. 



