02)ercula{?) in Carbon (ferous Limestone. 29 



We do not possess as yet any evidence to show the pre- 

 sence in our Carboniferous rocks of the tmbricated, unrjuiculatey 

 paucispiral, multispiral, and other forms into which the 0])er- 

 cula of Gasteropoda have been grouped. 



In our Sihirian rocks, on the other hand, we have the spiral 

 operculum in Euomphalus. 



The Naticopsis in the British-Museum collection is unfor- 

 tunately, except in the region of the mouth, imperfect. In all 

 probability it is N. Phitiipsii, M'Coy. The operculum is 

 oval, somewhat pointed in its upper part, and concave exter- 

 nally, the most depressed point answering to the internal 

 muscular scar. It bears the closest resemblance to those 

 opercula in situ from Law Quarry described in a preceding 

 paragraph. The concentric strise are very close and regular. It 

 fits the aperture of the shell exactly, and is more or less 

 thickened where it abuts against the pillar-lip (PI. II. fig. 8). 



2, Silurian Forms. 



The British-Museum collection contains two peculiar bodies 

 from the Upper Silurian beds of the island of Gothland, stated on 

 the labels forwarded with the specimens to be tlie opercula of 

 Euomphalus (PI. II. fig. 9). They are bluntly conical in form, 

 rising from a flattened or somewhat conca"(^e base. The smaller 

 is 3| lines high, by nearly 6 wide ; the larger one is 7 lines 

 in height, by a little more than 6 lines in diameter. Botli are 

 marked by close-set concentric raised lines of laminte, subim- 

 l>ricating upwards. About half the distance from the base 

 to tlie blunt apex the sides are somewhat constricted, and the 

 upper part is thus partially separated from the lower. The 

 flatly concave base is bounded by a marginal rim, and the centre 

 is occupied by a somewhat sunken coiled nucleus. These 

 bodies have a very peculiar appearance, and, if really opercuLa, 

 must have belonged to a strong massive shell. The upper 

 third or apical portion appears to be devoid of concentric lines 

 or ridges. 



The collection is also enriched by several specimens of a 

 Euomphalus with the operculum in place, from the Wenlock 

 Limestone, and by an operculum lying on the weathered sur- 

 face of the limestone, separated from the shell. The opercula 

 of these shells correspond generally with the description given 

 by Mr. Smithe, but differ in one or two minor particulars. 



The marginal outline is quite circular or round ; but as the 

 centre is approached the spiral rings become decidedly oval 

 (PI. II. fig. 13). The number of these rings in the best-pre- 

 served of our specimens is seventeen, and there were evidentl}' 

 a few more. There is, however, no trace of a nodose or bead- 



