A71 Opcrcuhim from the Lilydale Limestone. 153 



of the operculum of C. striatum, His., and of those of two otlier 

 undetermined species. The general type is quite similar to that 

 of this portion of the shell economy in Oriostofiia as figured by 

 Lindstrom. The Lilydale opercula more closely resemble those 

 described by Smithe and myself from the Wenlock beds, than 

 they do those from Gotland ; at the same time, trivial differences 

 which strike the eye on close examination, may ultimately prove 

 of wider significance. The cross section of an operculum given 

 by Smithe is most undoubtedly more akin to that of the Lilydale 

 specimens than a»e any sections which could be derived from the 

 Gotland examples. The latter are wholly piano-conical in section, 

 Smithe's Wenlock operculum is certainly plano-concave, whilst 

 the Lilydale forms, on the other hand, are either bi- concave, or 

 slightly plano-concave. 



It is possible that the shell spoken of by Mr. T. S. Hall as 

 Euomp/ialus may throw some light upon this subject, and I 

 should much like to be permitted to examine this specimen. 



A strange similitude to some of these opercula is seen in a fossil 

 from the Corniferous Limestone of Indiana, described by Mr. 

 S. A. Miller as a sponge, under the name of Cyclospongia discus* 

 It is " circular, button-shaped or discoid, and consisting of 

 numerous thin, calcareous lamina?, having a concentric structure 

 and filled with minute canals or interstices." The upper surface 

 is slightly convex, bearing numerous concentric lines, the under 

 side slightly concave, with a " broad, undefined, shallow furrow 

 near the circumference, and round depression in the centre." 

 Were it not for the minute canals I should be much tempted to 

 regard this object as an operculum. 



I am indebted to Mr. C. Hedley, F.L.S., for the accompanying 

 drawings. 



POSTSCRIPT. 



Through the courtesy of the Honorary Secretary of the Royal 

 Society I have been permitted to add some additional informa- 

 tion obtained since this paper was written, and kindly contributed 

 by Mr. G. B. Pritchard. The latter informs me that he has in his 



*?eveiitee:itli Report Geol. Survey Imliaiia, 1892, t.l., f. 8 aii.l 9. 



