6 MOLLUSCA FROM THE GREAT OOLITE. 



is more produced, and has more convexity than the other ; the substance of the test is 

 papyraceous, and the surface of the convex valve often displays markings, which prove that 

 for a considerable period these shells were attached to other bivalves by the surface of the 

 flat valve, but that valve has never actually been observed attached, and it is very com- 

 monly preserved with the outer surface destitute of any traces of having been adherent. 

 Judging therefore from the varying dimensions of the specimens, it does not appear that it 

 adhered at any particular stage of its growth, but that it was only occasionally attached. 

 From Placuna it is distinguished by the absence of internal diverging teeth ; it is never 

 auriculated, as in Posidonia, and the position of the hinge groove is very different ; in 

 Posidonia it forms a depression in the hinge plate, lengthened laterally, but in our genus 

 it is transverse. The form occurs throughout the Oolitic rocks of England, exemplified 

 by several species, which have usually been referred either to Anomia or to J^lacuna, an 

 erosion which not unfrequently occurs at the thinnest part of the valves where the 

 muscular impression is situated, having apparently been mistaken for the foramen of an 

 Anomia. 



PLACUNOPSIS JURENSIS, Eoem. Sp. Tab. I, fig. 8, 80 b. 



PLACUNA JUEENSIS, Roemer. Verst. Nord. Deutsch. Ool., p. 16, t. 16, f. 4. 

 ANOMIA JUEENSIS, Morris. Cat. Brit. Fos., p. 105, 1843. 



Testa orbicularis, irregulari, papyraced, sublamellosd ; valvd convexd, umbone odtuso, 

 depresso, sulmarginali ; lineis radiantibus nodosis, laminis concentricis impressis. Valvd 

 alterd planatd, umbone parvo depresso, lineis radiantibus undatis et tenuissimis. 



Shell orbicular, irregular, very delicate, somewhat lamellose ; convex valve with the 

 umbo, submarginal, obtuse, and depressed ; radiating lines knotted, fine, numerous, waved 

 and irregularly impressed with the concentric lamina?. The other valve flattened or 

 irregularly concave, its umbo small and depressed, the surface ornamented with numerous 

 irregular radiating knotted lines. 



In numerous instances this species attached itself by the flat valve to Pectens, Lima, 

 and Triffoniee, whose characteristic markings although scarcely, if ever, indicated on the 

 interior of either valve, appear distinctly impressed upon the outer surface of the convex 

 valve, almost obliterating the ornamented structure proper to the valve, so that the surface 

 of the Placunopsis seems like a delicate tissue or veil spread over the Trigonia or Pecten. 

 What renders this fact the more remarkable is, that the species of Lima, Pecten, and 

 Trigonia, are very abundant, and are invariably found free from other attached 

 shells. The valves of this delicate shell are abundant in the shelly beds of the Great 

 Oolite, and occur likewise, though more rarely, in the Fuller's-earth and Inferior Oolite 

 of Gloucestershire ; but care is required to detach specimens, as it breaks with any trifling 

 concussion. 



Localities. Minchinhampton Common and Bisley Common in the Great Oolite. 

 Leckhampton Hill and Nailsworth in the Inferior Oolite. 



