26 Mr. R. Etheridge, Jun,, on some Oasteropod 



less composed of horny material, strengthened by the addition 

 of calcareous matter. The inner surface carries a muscular 

 scar ; and the point from which the operculum commences is 

 termed the nucleus. It may fit the mouth of the shell with 

 accuracy ; or the entrance may be only partially closed by it ; 

 and, again, in some genera it is quite wanting. The operculum 

 may be said to be concentric when it increases equally all 

 round and the nucleus is central or subcentral — imbricated or 

 lamellar when growing on one side only and with a marginal 

 nucleus — unguiculate or claw-shaped, with the nucleus apical 

 or in front. The operculum is said to be spiral when it grows 

 only on one edge, revolving as it grows, and is always sinis- 

 tral in dextral shells ; when few-whirled the operculum is 

 said to be paucispiral, or subspiral when the turns are little 

 marked ; on the contrary, when the whirls are very nume- 

 rous it is said to be multispiral. One side has sometimes a 

 projection, in which condition the word " articulated " is used 

 to express the form. The operculum is present in some species 

 of a genus, absent in others ; and it is also indifferently horny 

 or shelly {Woodward)'^ . 



The description of opercula in the Gasteropoda of the older 

 rocks has not been of frequent occurrence, although a few 

 instances have been observed both amongst Silurian and Car- 

 boniferous representatives of the class. 



Amongst {Silurian univalves we meet with the peculiar 

 genus Maclurea, in which the operculum is thick, solid, and 

 sinistrally subspiral, with two internal projections, one of 

 them beneath the nucleus, very thick and rugose f. Mr. Salter 

 has given excellent figures of the operculum of both M. Logam, 

 Salter J, and M. PeacJm, Salter §, from the Durness Lime- 

 stone of the northern Highlands. 



In addition to those of Maclurea, the operculum of Euom- 

 phalus has been noticed by the late Dr. S. P. Woodward ||, 

 who describes it as shelly, round, and multispiral. 



Mr. F. Smithe, LL.D.^, has described and figured the 

 operculum of Euomphalus sculptus, Sow. He states that it 

 is shelly, ovate, concave within, plane without, thin, and with 

 a bevelled edge. The spiral consists of twelve whirls. 



Passing to rocks of the Carboniferous period, we find that 

 the operculum has been observed chiefly in the genus Nati- 



* Man. Mollusca, pp. 101, 102. 

 t Ibid. p. 202. 



X Murchison's ' Siluria,' 4tli ed. p. 197, foss. 40. fig. 1 a ; and dec. i. 

 Geol. Survey Canada, t. i. 



§ Quart. JoiUTi. Geol. Soc. xv, p. 378, t. 13. figs. 1 b, 3-5. 



II Man. Moll. p. 145. % I'roc. Cotteswold Nat. Field-Club. 



