210 THE LIAS AMMONITES. 



32. Tretoceras, Salter, 1859. 



Siph uncle small, almost central, bead-like ; the chambers surrotmding the siphuncle 

 pierced by a longitudinal tube ; Silurian. 



33. HuRONiA, Stokes, 1824. 



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 externally 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. = Melia, Fischer, 1829. = Thoracoceras, 



Eischer, 1844. 



Siphuncle sometimes large, lateral, not complex ; Silurian into Trias. 



5. Shell spiral, not in the same plane. 



39. Trochoceras, Barrande, 1847. = Semicornu, Klein, 1770. = Inachus, 



Hisinger, 1837. 



^ Turns of the shell at first in contact, not overlapping, afterwards generally spirally 

 divergent, occasionally tangentially divergent ; siphuncle mostly near the margin on the 

 outer (convex) side, edge of exterior chamber without fold ; Silurian. 



