eu OnnunD ORTHOCHOANITES 519 
sometimes forming more or less discontinuous longitudinal ridges. Silurian and 
Devonian. 
Ctenoceras, Noetl. Cyrtoceracones like Dawsonoceras dulce, Barr. sp., 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. Siphuncle dorsad 
of centre. Ordovician. 
Family 3. Kionoceratidae. 
_ Orthoceracones and cyrtoceracones with more or less well-murked 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, 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. 1064). Like the 
last, but with more or less spinous longitudinal ridges. Silurian 
to Carboniferous. 
Il. 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- oetocal 
tudinal striae or fine ridges, especially in the young, but these — ryoracoceras corbulqtum, 
generally disappear before the ephebic stage. Siphuncular seg- Barr. sp. Silurian (Btage 
: ; 5 E); Dvoretz, Bohemia (after 
ments slightly nummuloidal, fusiform, or tubular. Barrande). 

Family 4.. Tarphyceratidae. 
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, tubular and ventrad of centre. 
Aphetoceras, Deltoceras, Barrandeoceras, Tarphyceras, Hyatt ; Planctoceras, Eury- 
stomites, Schréder ; Falcilituites, Remelé. Ordovician. (For descriptions see Hyatt’s 
Phylogeny, 1894.) Ewrystomites and Tarphyceras are wholly nautilicones, 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 ephebie stage vt 7s tubular and dorsad of centre. 
Schroederoceras, Litoceras, Trocholitoceras, Hyatt; Trocholites, Conrad (Palaeo- 
nautilus, Palaeoclymenia, »Remelé). Ordovician.  Discoceras, Barrande. Ordovician 
and Silurian. 
