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DEVONIAN FAUNA. 



to the posterior wing, and sloping out flatly to the lower margins. Surface 

 covered with about sixty rather close, alternating, elevated, rounded rays, 

 separated by narrow furrows ; the whole crossed by occasional indistinct growth- 

 bulges and (in well-preserved specimens) by very minute and distant, regular, 

 elevated threads, and apparently by close, intermediate strise. 



Right valve transverse, convex, oblique. Umbo slightly more central and less 

 prominent than the umbo of the other valve. Front wing long, square, deeply- 

 notched below ; in contour, convex, and separated from the umbo by a deep con- 

 cavity. Hind wing flattish, well-defined. Contour of surface slightly convex in 

 the centre, very steep in front and behind, and spreading out flatly to the lower 

 margins. Surface bearing six or seven sharp, coarse, distant rays upon the hind 

 wing only, the rest of the surface having no rays ; but the whole being covered 

 by minute, distant, sharp, very regular, elevated threads, which become closer on 

 the front wing, and follow the curves of the margins. 



Size. — Left valve : Length, 23 mm. ; breadth, 18 mm. ; depth, 4 mm. A 

 small specimen of the closed valves measures 15 mm. long, 11 mm. broad, and 

 6 mm. deep. 



Localities. — Of the left valve there are seventeen specimens in my Collection and 

 four in the Woodwardian Museum from Lummaton, two probably from the same 

 locality in the Torquay Museum, and one from Barton in Mr. Lee's Collection. 

 Of the right valve there are three specimens from Lummaton in my Collection. 

 There is a specimen of the closed valves from Lummaton in the Torquay Museum. 



Remarks. — This species seems to be well-defined and clearly distinguishable 

 from the accompanying forms. It is characterized by its small size, its large 

 front wing, its close-set, rounded ribs, and its very distant, minute, and regular 

 transverse threads. The ribs seem coarser on the wings than on the central 

 parts of the shell, and the furrows increase in width as they approach the 

 margins. 



This shell presents some amount of variation both in shape, convexity, and 

 ornament. I formerly thought that several of the specimens {e.g. PI. VII, figs. 10 

 and 11) could be specifically separated, owing to their larger front wing, their 

 greater convexity, their more defined hind wing, and their closer and fewer ribs. 

 Further examination, however, makes me believe that these appearances are in 

 great part due to these specimens, though apparently well-preserved, really 

 wanting their marginal parts, and therefore that they can at most only be 

 regarded as varieties of the present form. 



The evidence that the right valves belong to the same species is only pre- 

 sumptive. My three examples of them are all so defective that their true shape 

 cannot be defined, but their ornament is just what might be expected in com- 

 parison with that of the left valves. The only specimen with the two valves in 



