THE CEPHALOPODA. 209 



Shell sliglitl}' 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. Cyrtocerina, 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. Streptoceras, Billings, 1866. 



Shell slightly curved, pear-shaped, edge of exterior chamber contracted to form three 

 lobes ; Silurian. 



29. Pii.ocERAS, 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. = SpiruHtes, Parkinson, 

 1811. = Clymenia {pars), d'Eichvi^ald, 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. 



30a. LiTUCNCULUs, 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, 

 Moutfort, 1808. = Achelois, Montfort, 1808. z= 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. 



