210 
THE LIAS AMMONITES. 
32. Tretoceras, Salter, 1859. 
Siphuncle small, almost central, bead-like ; the chambers surrounding 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. 
86. 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, Com^ad, 1847. = Melia, Fischer, 1829. Thoracoceras, 
Fischer, 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. =z 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. 
