28 Mr. R. Etlieridge, Jun., on some Gasteropod 



nucleus is nearest. The reverse side of the operculum is 

 plain except for a slig-ht convexity in the centre. As before, 

 Law Quarry (PI. 11. fig. 2). 



Another condition consists of elongated pinna-shaped bodies, 

 iiat or very slightly concave on one surface, and without 

 visible marks or ornament. The other aspect is convex, 

 particularly on one side, where it is thickened and gradually 

 thins away to the edge. One end is rounded, the other 

 bluntly pointed. As before, Law Quarry (PI. IL fig. 3). 



The fourth type consists of thin auriform bodies, a little 

 thicker on one of the longer margins than on the other, 

 concave on one side, more or less flattened on the opposite. 

 The latter carries a minute central depression or umbilical 

 spot, round which the surface is concentrically striated, and a 

 direct groove passing to the acuminated end. On the concave 

 side of these opercula there is a scar which may be generally 

 described as horsehoof-shaped, and a groove bounding the 

 raised portion. As before. Law Quarry, and on a similar 

 horizon at Liverteil Quarry near Kirkcaldy (PI. IL fig. 4). 



The fifth and last form is that of a small, thin, round oper- 

 culum, concave on one side, more or less convex on the 

 other. The concave side shows a doubly impressed scar, 

 having the appearance of a dumb-bell. As before, Law 

 Quarry (PI. IL fig. 5). 



The deposit yielding the opercula occasionally furnishes 

 examples of Gasteropoda with them in situ. I give two 

 illustrations of this, one with the operculum entire, the other 

 with it only partially preserved. In both cases the opercula 

 fit the mouths of the shells completely. In the former of 

 these there is a central depression, and the thickened side of 

 the operculum is placed against the pillar-lip of the shell, the 

 sharp edge being outward, resting against the outer lip. There 

 are faint signs of concentric striaj ; and the centre is depressed. 

 In all probability these are minute species oi Naticopsis (PL II. 

 figs. 6 & 7). 



The sum of the information deducible from the study of 

 these little objects appears to be, that, in the first place, certain 

 Carboniferous Gasteropoda possessed thick bony opercula, of 

 a more or less oval form, speaking generally, and of variable 

 size. 



In the second place, on the contrary, others were provided 

 with thin, round, or irregularly oval doors to their aper- 

 tures, and when any structure is visible it appears to be con- 

 centric. 



Again, a form of an entirely different shape is foreshadowed 

 by the elongated variety described as the preceding third type. 



