180 Messrs. Jones and Kirkby on the 



Langton, in Durham ; railway-cutting at Chapel Houses (west 

 of Pierce Bridge), Crakehall, Thornton Watlass, Nosterfield, 

 and Hampole, in Yorkshire. In Middle Magnesian Lime- 

 stone : Tunstall Hill, in Durham. In Upper Magnesian 

 Limestone : Byer's Quarry, and cliffs to the south of Marsden, 

 on the Durham coast. 



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

 (Plate III. fig. 2.) 



Beyrichia umbonata, D'Eichwald, 1860, Lethaea Rossica, livr. vii. 



p. 1347, pi. lii. fig. 10. 

 Kirkbya umbonata, Jones & 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 y% 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-Uke 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 



