ACTINOPTERIA FLUCTUOSA. 25 



the posterior end and the comparative dimensions cannot be ascertained. The 

 peculiar radiating ribs are represented inside the valve, but not so finely as outside. 



PI. IV, fig. 10, is a fine bivalve example from the Carboniferous Limestone of 

 Bskerhouse, near Grassington, almost complete, but the valves have slipped slightly 

 over each other. The specimen is in the collection of the Geological Survey, 

 Jermyn Street, and shows the more correct contour of the valve, and also that the 

 umbones are terminal in the testiferous fossil, but subterminal in casts of the 

 interior. 



This species is most commonly found in beds of shale, but when it does occur 

 in limestone attains a very large size. 



I consider that P. regularis, Etheridge, jun., was probably a weathered cast of 

 A. per sulcata. 



P. sulcafus, as figured by M'Coy, is more transversely triangular, and the ribs 

 are finer and straighter, but I believe that this species was founded on the upper 

 fragment of A. per sulcata. 



AcTiNOPTEEiA FLUCTUOSA, R. Etkerhhjc, jiiii., sp., 187o. Plate V, figs. 8 — 12. 



Pteeonites fluctuostjs, B. Etheridge, jun., 1873. Geol. Mag , vol. x, p. 3-45, 



pi. xii, fig. 1. 

 — — B. Etheridije, 1S88. Brit. Foss., pt. 1, Palaeozoic, p. '270. 



Specific Gharacters. — Shell small, transversely trigonal, gibbose. The anterior 

 end is very short, almost pointed. The lower border descends downwards and 

 backwards, becoming curved and broadly rounded into the posterior border, which 

 is very slightly convex. The hinge-line is straight. The umbones are small, 

 gibbose, pointed, raised, placed far forwards, but not terminal. The valves are 

 obliquely swollen, l)ut become expanded and compressed posteriorly. The posterior 

 wing is well marked and compressed. 



Interior unknown. 



Exterior. — The umbonal region is marked l)y concentric lines of growth, and is 

 more or less smooth, but nearer its margin the concentric lines are decussated by 

 well-marked radiating ribs, which are often flexuous. The anterior portion of each 

 valve is free from radiating ribs. Posterior wing marked by straight, strong, 

 radiating ribs. Shell thin. 



Dimensions. — PI. V, fig. 8, the type of R. Etheridge's species, measures — 

 Antero-posteriorly . . . .9 mm. 



Dorso-ventrally . . . .0 mm. 



Localities. — Scotland : Crossbank, Crossford ; Cleckliinon, Kilbride ; Cani})sie, 



4 



