ORBICULA. 9 



often difficult to distinguish and describe them. Professor Phillips states this species to 

 occur in the inferior Oolite of Yorkshire ; it has also been found in the marlstone near 

 Bathford, during the cuttings for the Great Western Railway, by Mr. Walton, whence 

 the fine internal specimens figured in Plate I, figs. 1, a, b, c, d, were obtained; these 

 figures are enlarged. 



I do not know any other English Oolitic species ; fragments of a Lingula have been 

 found by Mr. Moore in the upper lias of Ilminster, but not sufficiently perfect to be 

 described or identified. 



Figs, la, lb, represent the larger valve enlarged, with the groove for the passage of the 

 muscular peduncle. 



Pigs. 2c, 2d, smaller valve enlarged. It will be seen by these figures the muscular 

 impressions vary slightly in different specimens. 



Genus — Orbicula, Cuvier. 1798. 



Shell inequivalved, more or less orbicular, upper valve conical, with apex nearer the 

 posterior margin, smaller valve depressed, flat, or slightly convex, affixed to submarine 

 bodies by a tendinous pedicle issuing through a fissure, varying in length and size, 

 extending from its centre to near the margin ; no hinge, or calcareous shelly supports ; 

 structure horny. 



Obs. We are only acquainted with three British Oolitic Orbiculae, viz., 0. Toivnshendi, 

 reflexa, and Humphresiana ; 0. Toivnshendi is the largest orbicula known. Two other 

 shells have been placed among the orbicula : 0. granidata, Sow., M. C, Tab. 506, fig. 34, 

 stated to be from the great Oolite of AnclifF, and 0. radiata, Phillips, Geol. of York, 

 Tab. 4, fig. 12, as from cor. Oolite of Malton ; these, however, do not seem to belong to 

 the class of Brachiopoda, but to that of Gasteropodes. 



2. Orbicula Townshendi, Forbes, MS. Plate I, figs. 2, 2a, 2b. 



Diagnosis. Shell bivalve, thin, almost circular, upper valve very convex, regularly 

 rounded; apex near the posterior margin, greatest elevation of the valve towards the 

 central part, the apex lying considerably lower; surface smooth, horny, with irregular 

 circular lines of growth ; inferior or attached valve, slightly concave, with deep depression, 

 extending and widening from the centre to within a short distance from the posterior 

 margin, leading to a long, wide, ovular fissure, from which the peduncular fibres issued, 

 and which must, in this species, from the depth of the valve, have been of some length, 

 the fissure measuring 5 lines in length and 3 in breadth, and extending to within 

 4 lines of the margin of the shell. This valve is ornamented by numerous and regular 

 slightly-elevated concentric striae, not all forming the complete circle, sometimes extending 



c 



