LEPTODESMA. 119 



This suggested genus appears to approach Gassianella in shape, but is more 

 oblique, and the right valve is deeply convex, while probably the other valve is 

 much flatter. Many points must for the present remain undefined, and the genus 

 must therefore be regarded as entirely provisional. It seems, however, to be 

 essentially aviculoid. Its angularity of contour reminds us of Conoca/rctium, 

 but it has a definite wing in place of the tubular projection of that genus. 



1. Cobbaoephalus angulosus, n. sp. Plate XIII, figs. 9, 9 a. 



Description. — Shell small, convex, angulated, very transverse and oblique. 

 Umbo prominent, elevated but flattened, much incurved, arching forward, acute, 

 situated at or about the anterior third of the length, and apparently extending 

 slightly above the hinge-line. Hinge-line equal to the greatest length of 

 shell. Contour of surface vertically convex ; horizontally angulated; being (1) flat 

 on a narrow area bounded by rounded angles, which run from the umbo obliquely 

 backward to inferior margin ; (2) in front of this, deeply perpendicular, then 

 oblique at the base of front wing, then flat on front wing, and lastly sigmoid at 

 the antero-superior corner; (3) behind the central band, first perpendicular, then 

 oblique, then angulated by a line from behind umbo to the postero-inferior corner, 

 and then oblique and concave, forming a broad hind wing. Anterior wing large, 

 trapezoidal, pointed in front, bounded by an angulated margin. Anterior margin 

 apparently nearly perpendicular below the wing. Inferior margin very short and 

 straight. Posterior margin oblique, convex, angulated. Surface covered by very 

 numerous and crowded, parallel but unequal, sharp, elevated, concentric threads 

 or striee, separated by deep concavities, which are rather wider than the threads, 

 and crossed by numerous indistinct radiations. 



Size. — Height 10 mm., length 14 mm., depth 2*5 mm. 



Localities. — There is a single specimen from Top Orchard in the Woodwardian 

 Museum. 



Remarks. — I have only seen a single valve of this strange shell. 



2. Genus— Leptodesma, Hall, 1883. 



The distinguishing mark of this genus, as compared with Leiopteria, is its 

 nasute, angulated front wing. It also appears more oblique in general shape, the 

 anterior margin being less protruded. Though some species seem difficult to 

 assign, the two genera appear on the whole to be well characterised. 



