Kirkbya umbonata (D ’Eichwald), and var. radiata,, 1867

2. Kirkbya umbonata (D ’Eichwald), and var. radiata, nov.

(Plate III. fig. 2.)

Beyrichia umbonata, D ’Eichwald, 1860 , Lethrea Rossica, livr. vii. p. 1347, pl. lii. fig. 10.

Kirkbya umbonata, Jones A Kirkby, 1867 , Trans. Geol. Soc. Glasgow, vol. ii. p. 221; and 1871, vol. iii. Suppl. p. 29.

Kirkbya umbonata, Armstrong and others, 1876, Catal. W.-Scot. Foss, p. 45.

Ark-shaped, with a central umbo and a wide ventral frill or marginal radiated expansion to each valve; height about half the length or more. Dorsal border straight; ventral border usually boldly curved; extremities rounded, the poste ­ rior being the larger and more obliquely curved than the other. The umbo is near the dorsal border, oval in shape, about a third of the valve ’s length and half its height in size; it is often awry or placed obliquely on the valve. There is a lamellar expansion or radiate frill round the ventral and extreme borders of each valve, which in older specimens has considerable development; these frills are plaited or ruffle ­ like, and have more or less scalloped edges. The surface is reticulated in some specimens, and in others finely papulose. Length 1/2 3 inch.

This species is subject to some variation. Younger indi ­ viduals more especially look different, owing to greater regu ­ larity of outline, the smaller size and pap-like form of the umbo, and the absence or meagre development of the frills. In the Russian specimen, described and figured by D ’Eich ­ wald, from the Yellow Carboniferous Shale of Sloboden, Government Toula, the surface is said to be finely striated and pitted, the space between the umbo and the margin is raised into nearly regular concentric rolls or rounded ridges, and the frilling is absent, as in some of our specimens.

It is undoubtedly a near ally of K. permiana , with which it has much in common. It is usually, however, to be distin ­ guished from the latter by its greater relative height, the more Leperditia - like truncation of the valves, by its irregular hump or umbo, and by the character of its ventral rims. The sur ­ face-reticulation also differs somewhat, not being so definitely meshed like network. The frill-less form occurs in Scotland and in Westmoreland, and it has the umbo quite regular sometimes. The frill may be obsolete, or easily lost perhaps, as in the fringed Beyrichiae.

K. umbonata occurs in the following localities:—

Scotland. Carboniferous Limestone (Lower): Brockley and Hairmyers, in Lanarkshire; Craigenglen, in Stirling ­ shire; Whitefield and Carlops, in Peeblesshire; Billhead Quarry, near Cockmuir Bridge, in Edinburghshire; Kidlaw Quarry, Cateraig, Burlage Quarry, East Salton, in East Lothian; Wilkicson, Ladedda, Charleston, Cowdens Quarry, coast east of St. Monans, in Fifeshire.

England. Carboniferous-Limestone series: Scremerston, Barmoor Bedhouse (Lowick), Ridsdale, in Northumberland; Calces, in Cumberland. In the Scar Limestone: at Arnside and in a railway-cutting near Heversham, Westmoreland.