CRANIA. 195 



reproduced all the inequalities of its surface, but with age, and from the shell acquiring 

 greater thickness, these inequalities were generally levelled. Nor is it an uncommon cir- 

 cumstance to find the roughness or sculpture of the object to which the lower valve adheres 

 likewise reproduced upon the outer surface of the upper or unattached valve, in a similar 

 manner to what we find to be the case with certain species of oyster. It is no easy matter 

 to distinguish certain forms of Crania, as several species bear so close a resemblance to each 

 other, and it was not until I had been able to study the original type of Prof. M'Coy's 

 Orbknla quadrata (kindly communicated by Sir R. Griffith) that I could identify it with the 

 same form which occurs so abundantly in the Carboniferous shales of Scotland. 



In England but few examples have been hitherto discovered ; one or two specimens 

 were found by Mr. C. Moore, at Holwell, near Erome. In Scotland it occurs at Gare, 

 in Lanarkshire, at 239 fathoms below " Ell Coal," 343 at Langshaw Burn, and 375 at 

 Kilcadzow. It occurs also at Auchentibber and Calderside, High Blantyre ; Capelrig, 

 East Kilbride ; Brockley, near Lesmahago ; and Robroyston, north of Glasgow. In Ayr- 

 shire, at West Broadstone, Beith ; Goldcraig, near Kilwinning ; Cessnock, near Galston ; 

 and on the bank of the stream Pomillen, near Strathavon. In Renfrewshire, at llowood, 

 near Paisley ; and Orchard Quarry, Thornliebank. In Kircudbrightshire, in strata cropping 

 out on the seashore, near Kircudbright. In Stirlingshire, in the Balglass Burn beds, and 

 in those of the Campsie main limestone. In Ireland it was found in Carboniferous shale 

 at Rahan's Bay, in Donegal, one mile south-west of Dunkineely village. On the Continent 

 it occurs at Tournay, in Belgium ; and at Tuscombia, Alabama, in America. 



Crania Ryckholtiana, Be Koninck. PI. XLVIII, figs. 15, 16, 17? 



Patella Ryckholtiana, Be Kon. Animaux Foss. de la Belgique, pi. xxiii, fig. 5, 



1843. 

 Crania vesicularis, M'Coy. Synopsis of the Char, of the Carb. Foss. of Ireland, 

 pi. xx, fig. 3, 1844. 



Spec. Char. Ovato-orbicular, nearly circular, or slightly longer than Avide. Upper 

 valve conical, limpet-like, the vertex being sub-central and closer to the posterior than to 

 the anterior margin ; surface smooth, marked only by a few concentric lines of growth ; 

 lower or attached valve unknown. Dimensions variable ; two British examples have 

 measured — 



Length 8, width 10, depth 6 lines. 



Obs. It appears quite probable that the Patella RycMoltiana and Crania vesicu- 

 laris belong to the same species, but unfortunately, although several exteriors of the 

 upper valve have been discovered, no example showing the interior has hitherto turned up. 

 In England it has been found in the Carboniferous limestone of Castleton, in Derbyshire. 



