36 



3. Shell a combination of curved and straight lines in the same plane. 



30. Lituites, Breynius, 1732 Turns of the shell at first generally in con- 



= Hortolus, Montfort, 1808. tact, afterwards tangentially divergent; 



= Spirulites, Parkinson, 1811. siphuncle variable; edge of exterior 



= Clymenia (pars), Eichwald, 1842. chamber contracted; Silurian. 

 = Ancistroceras (pars), Boll, 1857. 

 30a. Lituuncuiajs, Barrande, 1867. Same as Lituites, but with the edge of the 



exterior chamber contracted, the form . 



ideal, not yet discovered in rocks ; 



Silurian ? 



4. Shell straight. 



31. Orthoceras, Breynius, 1732 Siphuncle small, almost central, bead-like ; 



= Molossus, Montfort, 1808. aperture of exterior chamber open, 



=Echidnis „ „ without folds ; from Silurian into Trias. 



=Achelois „ „ 

 = Hyolithes, Eichwald, 1842. 

 =Koleoceras, Portlock, 1843. 

 = Cycloceras, McCoy, 1844. 



32. Tretoceras, Salter, read 1856, 1858 Siphuncle small, almost central, bead- 



like ; the chambers surrounding the 

 siphuncle pieced by a longitudinal tube ; 

 Silurian. 



33. Huron i a, Stokes, 1840 Siphuncle large, central, the upper part of 



each division expanded and connected 

 with a small central tube by radiating 

 plates ; Silurian. 



34. Actinoceras, Stokes, 1840 Siphuncle large, central, the middle part 



of each division expanded and connected 

 with a small central tube by radiating 

 plates ; Silurian into Carboniferous. 



35. Ormoceras, Stokes, 1840 Siphuncle large, almost central, bead-like, 



the middle of each division contracted ; 

 Silurian into Devonian. 



36. Aulacoceras, Hauer, 1860 Siphuncle small, lateral, plain ; shell exter- 



nally corrugated thickening towards the 

 base. Trias. 



37. Endoceras, Hall, 1847 Siphuncle large, lateral with cone-like 



partitions, inserted in each other, their 

 points being directed towards the small 

 or earliest portion of the shell ; Silurian. 



38. Cameroceras, Conrad, 1847 Siphuncle sometimes large, lateral, not 



= Melia, Fischer, 1829. complex; Silurian into Trias. 



= Thoracoceras, Fischer, 1844. 



5. Shell spiral, not in the same plane. 



39. Trochoceras, Barrande, 1847 Turns of the shell at first in contact, not 



= Semicornu, Klein, 1770. overlapping, afterwards generally spi- 



= Inachus, Hisinger, 1837. rally divergent, occasionally tangenti- 



ally divergent; siphuncle mostly near 

 the margin on the outer (convex) side, 

 edge of exterior chamber without fold ; 

 Silurian. 



40. Hercoceras, Barrande, 1865 Turns of the shell at first in contact, 



slightly overlapping, afterwards spirally 

 divergent ; siphuncle marginal on outer 

 side, edge of exterior chamber contracted 

 and compressed transversely ; Silurian. 



