THE CEPHALOPODA. 
209 
Shell slightly curved, never forming a complete revolution ; siphuncle variable in 
position, generally on convex side ; exterior chamber open, its edge not contracted ; from 
Silurian into Devonian. 
26. Ctrtocerina, Billings, 1866. 
Shell like Cyrtoceras, but shorter and thicker ; Silurian. 
27. Oncoceras, Hall, 1847. 
Shell slightly curved, pear-shaped ; siphuncle lateral, edge of exterior chamber 
contracted in the middle ; Silurian. 
28. Sti^eptoceras, Billings, 1866. 
Shell slightly curved, pear-shaped, edge of exterior chamber contracted to form three 
lobes ; Silurian. 
29. PiLOCERAS, Salter, 1859. 
Same as Cyrtoceras, but the septa forming a series of conical partitions, pointing 
towards the smaller (earliest) part of the shell ; Silurian. 
3. Shell a combination of curved and straight lines in the same plane. 
30. Lituites, Breynius, 1732. = Hortolus, Montfort, 1808. = Spirulites, Parkinson, 
1811. = Clymenia {pars), d'Eichwald, 1842. = Ancistroceras (pars). Boll, 1857. 
Turns of the shell at first generally in contact, afterwards tangentially divergent ; 
siphuncle variable ; edge of exterior chamber contracted ; Silurian. 
SOa. LiTUUNCULiJs, Barrande, 1867. 
Same as Lituites, but with the edge of the exterior chamber not contracted, the form 
ideal, not yet discovered in rocks ; Silurian ? 
4. Shell straight. 
31. Orthoceras, Breynius, 1732. = Molossus, Montfort, 1808. = Echidnis, 
Montfort, 1808. = Achelois, Montfort, 1808. = Hyolithes, d'Eichwald, 1842. = 
Koleoceras, Portlock, 1843. = Cycloceras, M'Coy, 1844. 
Siphuncle small, almost central, bead-like ; aperture of exterior chamber open, with- 
out folds ; from Silurian into Trias. 
