AVICULOPECTEN. 129 



2. Avicdlopectbn nexilis, Sowerby, sp. Plate XVI, figs. 5, 6, and Plate XVII, 



figs. 2, 3. 



1840. Pecten nexilis, Sowerby. Geol. Trans., ser. 2, vol. v, pt. 3, pi. liii, 



figs. 1, 2. 

 1891. Aviculopecten aquisgeanensis, Freeh. Abliandl. Geol. Specialk. Preuss., 



Band ix, pt. 3, p. 19, pi. i, figs. 1 — 1 b. 



Description. — Shell small, rather flat (the right valve being apparently almost 

 flat and the left gently convex), subcircular, probably very slightly oblique, nearly 

 equilateral. Wings small, flat, triangular, broad, nearly equal, bounded by 

 sigmoid margins, defined (especially the anterior wing) by strong straight sulci, 

 which reach the margins. Lateral margins, deeply concave under the wings, and 

 then becoming deeply convex in the lower parts, the posterior corner being slightly 

 more produced. Inferior margin nearly semicircular. Hinge-line straight, rather 

 more than half the length of the shell. Umbo of the right valve small, sharp, 

 central, proximate to the hinge-margin ; umbo of the left valve rather larger, 

 deeper and more rounded. Contour of back slightly convex, deepest near the 

 umbo, spreading out flatly to the margins. Surface covered with about fifty 

 fine, sharp, minute, elevated, distant, alternating ribs, about half of which have 

 divaricated from the centre ; the whole crossed by equally numerous but still finer 

 and smaller parallel concentric threads, which extend also over the wings. 

 Wings with about seven radiating rays similar to those on the body of the 

 valves. 



Size. — 16 mm. long, 17 mm. high, and 4 mm. deep (left valve). 



Localities. — In the Museum of the Geological Society is Sowerby's type from 

 Barnstaple ; in the Barnstaple Athenaeum, two specimens from Top Orchard and 

 one from Braunton ; in Miss Partridge's Collection, one from Upcott Arch Quarry ; 

 and in my Collection, several specimens from the Ostracod-bed close to the Laticosta 

 bed at Baggy. 



Remarks. — For a long time I was only acquainted with very imperfect 

 specimens of this shell ; but recently I have found it occurring in abundance 

 in the lower beds of Croyde Bay. There is no doubt of the identity of these 

 fossils with the species as described by Sowerby. 



Avlculopecten aqulsgranensls, Freeh, appears exactly identical in every respect. 



Affinities. — It is distinguished from A. Hallli by its very much finer and more 

 numerous ribs. 



R 



