Aji Opercnluui from the Lily dale Limestone. 151 



like lines, the edges of the component laminae. Every here and 

 there one is larger than the others. In sections prepared for the 

 microscope, the concentric laminaj become very apparent, both in 

 horizontal and vertical sections. The latter also display the 

 outline exceedingly well. The opercula are practically round, a 

 fact which can be easily ascertained by following the concentric 

 laminae in a transverse section. This point is an important one, 

 as it m.ay bear on the question of the identity of the operculum 

 to its shell. 



Mr. T. S. Hall mentioned in the discussion before referred to, 

 that an operculum had been found "wedged into the mouth of an 

 Euomphalus." Not having seen this genus amongst the 

 Lilydale fossils, I am unable to follow the suggestion further, but 

 must fall back upon the question — Do these opercula appertain 

 to either of the described shells, Oriostoma Northi, or one of the 

 Cyclonemce ? The mouth in the former is not, strictly speaking, 

 round, but angulated towards the inner lip. On the other hand, 

 Cyclonema australis, and probably also C. Ulydalensis possess a 

 round mouth, but this difficulty presents itself — the smallest 

 operculum before me is too large for the largest C. aicsiralis. It 

 is hardly necessary to consider such a form as Phanerotrema 

 australis, that being a member of the Pleurotomariida?, in which 

 the operculum is corneous ; but, at the same time, so far as mere 

 size goes, the mouth of this species would far better accommodate 

 a body of the size of these opercula than the shells mentioned 

 above. If one may be perniitted to surmise that the Euomphalus 

 mentioned by Mr. Hall be Oriostoma A'orthi, then the matter 

 narrows itself down to the question, is the operculum in such a 

 position that it can be regarded as in situ ? If on the other hand 

 the shell be Euomphalus tlie matter becomes still jiiore interesting. 

 Let us now consider what previous investigations on Silurian 

 opercula teach us. 



Many years ago Dr. F. Sraithe, M.A., figured* the well-known 

 Wenlock shell, Oriostoma sculptum, Sby. sp., with its operculum 

 /;/ situ. The latter is plano-concave, plane without, concave 

 within, formed of twelve concentiic lamina?, and with a well- 

 marked nucleus and bevelled edges ; therefore, except in its 



* Observations on tlie Opercula of some Silurian Gastropoda.— Proc. Cotswolil Kat. Field 

 Club. 



