138 DEVONIAN FAUNA. 



oblique. Inferior margin rather wide, convex, oblique. Postero-inferior margin 

 widely rounded or semicircular. Posterior margin oblique, nearly straight. 

 Surface covered with about twenty-three large, elevated, very rounded ribs, between 

 which is a single similar series of much smaller ribs, separated by concave inter- 

 spaces equal to the smaller ribs in width ; the whole crossed by minute and 

 indistinct close concentric stria?. Contour (in old forms) probably very steep, 

 becoming almost perpendicular in front. Right valve apparently very similar to 

 the other valve, very convex, with a large expanded wing in front. 



Size. A small left valve is 24 mm. high by 20 mm. long. A larger valve is 

 about 38 mm. high by 32 mm. long, and 8 mm. deep. 



Locality. There are two specimens in the Woodwardian Museum from 

 Barnstaple, and another in the Museum of Practical Geology from Croyde. 



Remarks. All these three specimens are defective, and, though they clearly 

 show the general character of this handsome species, are difficult to interpret in 

 detail, and therefore the above description must be regarded as in a measure 

 tentative. 



The best specimen is a small, flattish left valve in the Woodwardian. This 

 shows the small anterior wing, above which is seen the straight hinge (about 

 5 mm. wide), bearing three or four parallel ligamental furrows. With this 

 fossil appears to agree the Croyde specimen, except that it is much more convex, 

 and that the anterior side seems much more rounded, as though the anterior 

 wing, which is gone, were much smaller. The other Woodwardian specimen is 

 so distorted by pressure that it is impossible to make out its original contour ; 

 but its general shape and the large size of the ribs on the remaining wing (which 

 is, however, evidently very much distorted) may possibly point to its being the 

 opposite or right valve. 



Affinities. It may be near Pecten alternatus, Phillips, 1 from South Petherwyn, 

 but that shell is said to be not oblique, and is smaller, and has fewer, more 

 elevated, closer, and more definitely alternating major ribs, while there are no 

 signs of the minor series. Whatever, therefore, may be the shell which Phillips' s 

 figure represents, there is at present no sufficient reason to identify it with the 

 present species. 



Avicula rudis, Phillips, 2 widely differs in having a very much more finely 

 radiated left valve and a concentrically marked right valve. 



1 1841, Phillips, ' Pal. FOBS.,' p. 47, pi. xxi, fig. 78. 



2 1841, ibid., p. 50, pi. xxii, figs. 85 a, 5. 



