1 o2 Proceedings of the Royal Society of Victoria. 



section, remarkably like the opercula from Lilydale. Tn 1881 I 

 figured* some of these bodies from the- Carboniferous and 

 Wenlock rocks of Great Britain, and the Weiilock of Gotland. 

 Those from the English Wenlock occupy the mouths of 

 Oriostoma sciilpitifn, Sby., and are depressed-conical, circular 

 bodies, bearing seventeen or more concentric rings. The other 

 side is flattened near the margin and then rises at the centre into 

 a low spiral eminence. Allowing for the state of preservati(jn, 

 and slight variability, these agree perfectly well with those 

 figured by Mr. Smithe and also those now about to be 

 referred to. 



By far the most complete set of Silurian opercula figured, 

 however, are those from the Wenlock rocks of Gotland, by Dr. 

 G. Lindstrom.f He gives illustrations of those of Oriostoma 

 coronattiiii, Linds., and O. globosiiin., Schl., besides a number of 

 others not relegated at the time he wrote to their pi^oper 

 species. The whole of these are conical, in a greater or lesser 

 degree, and are thus described: — "The operculum, i.e.^ of the 

 genus, is calcareous and solid, on the inner side smooth with a 

 thick, elevated rim round the margins, outside conical, sometimes 

 higher than broad, covered with a number of spiral coils, 

 ornamented with exceedingly thin lines."J It will be at once 

 apparent that the opercula from Lilydale differ from those of the 

 Gotland molluscs in the entire absence of any conical outline ; on 

 the contrary, they are flattened disk-shaped. Liudstrom figures 

 one of the less conical filling the aperture. The variation in form 

 is very remarkable, from a depressed conical, through a depressed 

 roundly-conical, to an elongately-conical, or absolutely plug- 

 shaped outline, much resembling some rifle bullets. At the same 

 time all possess the flat or very slightly concave inner face, 

 accompanied by the external concentric coils, the latter having a 

 more or less subimbricating appearance. 



The operculum of Cyclonema is thus described : — " The 

 operculum is bioadly conical, with some ten large coils outside 

 impressed by a shallow groove along their superior border, and 

 streaked by oblique, transversal lines. "§ Illustrations|| are given 



» Ann. Mag. Nat. Hist., 1881, vii. (5), p. 29. 



tTlie Silurian Gastropoda and Pteropoda of Gotland, 1884. t. 17. 



X Ibid., p. 156. J Luc. cit., p. 174. |1 Ibid., t. 17. 



