122 DEVONIAN FAUNA. 



subtrapezoidal or tongue-shaped, bounded by a sigmoidal margin. Hind wing 

 very narrow, triangular, bounded by a concave margin. Front margin very 

 oblique, convex on the wing, concave at the base of the wing, and then gently 

 convex below the wing, passing insensibly into the long, slightly convex, and 

 oblique inferior margin. Postero-inferior corner very greatly produced, narrow 

 and deeply convex. Posterior margin very oblique, slightly concave, with a 

 curvature increasing upwards, meeting the hinge-margin at a right angle. 

 Contour of shell gently convex, but steep posteriorly behind the line from umbo 

 to the postero-inferior corner. Surface of back and wings covered by crowded 

 minute, regular, parallel, distant, sharp concentric lines, and by a few irregular 

 prominent growth-bulges. 



Left valve differing from the right by having its sharp umbo rather elevated 

 above the hinge-line, and its anterior wing convex and less defined. 



Size. — Height 13 mm., length 30 mm., depth of both valves about 4 mm. 



Localities. — In the Museum of Practical Geology is a slab (audits counterpart) 

 containing the two valves in close proximity from Braunton ; and in the Porter 

 Collection is a specimen of the joined valves from Pilton, a left valve from 

 Ptoborough, and another from Poleshill. 



Remarks. — While not in actual contact, the two valves from Braunton 

 doubtless belonged to the same animal; though, lying in different directions on a 

 slab that has suffered contortion, their dimensions are rather different. The 

 minute concentric ornament is remarkably regular ; over the posterior slope its 

 threads become rather stronger and more distant. 



Affinities. — Avicula innotata, Barrande, 1 is a shorter shell with a longer hind 

 wing and different ornament. 



4. Leptodesma anatinum, n. sp. Plate XIV, figs. 4 — 6. 



?18S9. Avicula, sp., Kayser. Abhandl. Konig. Preuss. Geol. Landes., n. s., pt. 1, 



p. 19, pi. vii, fig. 9. 



Description. — Left valve moderate in size, subquadrate, oblique, transverse, 

 moderately convex. Umbo rather prominent, obliquely incurved, proximate, 

 extending slightly above the hinge-line, and situated at the anterior third of its 

 length. Anterior wing large, broad, subtriangular, and defined by a distinct, 

 nearly vertical line from the front of the umbo. Posterior wing large, flattish, 

 triangular, defined, not reaching quite so far back as the postero-inferior point. 

 Anterior margin oblique, slightly incurved under the wing, and fuller below. 



1 1881, Barrande, ' Syst. Sil. Soheme,' vol. vi, pi. ccxxix, figs. 11, i_5, Et. E. 



