120 DEVONIAN FAUNA. 



1. Leptodesma optimum, 1 n. sp. Plate XIII, figs. 7, la. 



Description. — Shell inequivalve. Left valve large, oblique, sub triangular, 

 rather transverse, moderately convex. Umbo small, elevated, oblique, proximate, 

 arching forward, overhanging the hinge-margin, apically acute, and situate at 

 about one-fifth the length of the shell from its anterior end. Hinge-margin long, 

 straight, equal to the greatest length of the shell. Anterior wing rather large, 

 nasute in front, vertically convex, horizontally sloping, bounded by an oblique 

 linear depression, which reaches the margin rather more than halfway down. 

 Front cardinal angle between 60° and 90°. Anterior margin oblique, convex on 

 the higher part of the wing, then slightly concave to the end of the wing, and 

 then slightly convex as it passes into the short inferior margin. Postero- 

 inferior corner broadly and deeply convex. Posterior margin slightly oblique, 

 straight above, and gently convex below. Hind wing large, obliquely flat, 

 with a postero-superior angle of about 100°, and defined by a straight oblique 

 line from behind the umbo meeting the posterior margin not quite halfway 

 down. Contour of surface transversely convex across the line of greatest elevation, 

 which runs near the front side from the apex to the postero-inferior corner in 

 a gently sigmoid sweep, the convexity increasing in front of it and gradually 

 diminishing in rear of it. Shell marked by a few indistinct concentric growth- 

 bulges. 



Size of Left Valve. — Height 45 mm., length 57 mm., depth, 7 mm. 



Locality. — Two specimens from Marwood and one from Barnstaple are in the 

 Museum of Practical Geology, and oue small specimen from Barnstaple in the 

 Woodwardian Museum. 



Remarks. — It has the appearance of being decidedly inequivalve. The 

 specimens are almost in the condition of casts, lying in micaceous, ferruginous 

 matrix, and having the surface destroyed. The margin does not lie in oue plane, 

 but is inversely sigmoid in front profile. The Woodwardian specimen is very 

 much smaller than the others, but otherwise agrees with them, and shows the 

 front wing to be nasute. 



Affinities. — To Leptodesma potens, Hall, 2 it bears some resemblance in general 

 shape, though differing in being more acute below and less concave posteriorly. 



1 Citerior, citimus. 



2 1884, Hall, ' Pal. N. ¥.,' vol. vi, pt. 1, p. 188, pi. xxi, figs. 21, 30 ; pi. xxii, figs. 11, 12, 19—21 

 and pi. lxxxix, fig. 7. 



