Art. X. — An Operaihnn from the Lilydale Limestone. 

 (With Plate IX). 



By R. Etheridge, Jimr., Corr. Member. 



[Read 14tli September, 1893]. 



The opercula of Univalves are amongst the less common 

 fossils met with in rocks of Upper Silurian age, but although 

 known to occur in those of Europe, have not been described, so 

 far as I am aware, from deposits of a similar nature in Australia. 

 My acquaintance with opercula from the Lilydale Limestone was 

 first made through the collection of Mr. G. Sweet, of Brunswick, 

 and subsequently by means of collections made at Lilydale by 

 Mr. A. J. North, on behalf of the Australian Museum, Sydney. 

 These bodies were also casually referred to by Messrs. G. B. 

 Pritchard and T. S. Hall* during the discussion on the Rev. A. 

 Cresswell's paper f " Notes on the Lilydale Limestone." The 

 observations in question will be referred to later. 



The Opercula are disc-shaped, amphiccelous, strongly reminding 

 one of the vertebral centrums of some fish. Those I have seen 

 vary in size from half to once inch in diameter, and are bevelled 

 from the exterior inwards along the sides. Further, they are 

 thick solid bodies, almost equally concave, but the concavity less 

 acute on the exterior, and more gradually inclined inwards than 

 on the interior. The periphery of the latter side is flattened, the 

 central area small, depressed, and circular, and often presenting 

 a minute central nucleus. The thickness on the sides of the 

 largest example I have seen is two-eights of an inch, or a trifle 

 over ; the thinnest, three-sixteenths of an inch. Mr. Pritchard 

 informs me that he possesses examples of these opercula varying 

 in size from one-sixteenth to one and a quarter inches in 

 diameter, and from one-fiftieth to one-quarter of an inch in 

 thickness. The structure is very apparent, even to the 

 naked eye, the exterior exhibiting close concentric thread- 



» Proc. R. Soc. Vict., 1893, v. (n.s.), p. 260. t Ihid, p. 38. 



