196 



Accremental striae irregular in distance and size mark the 

 surface, and form rude flat concentric ridges at intervals. 

 Well-marked radial striae cover all the posterior half of the 

 shell, extending from the umbo to the ventral border, some- 

 what interrupted at the edges of the concentric ridges ; these 

 are quite absent from the anterior half. 



Dim. — Antero-posterior diameter, 8'9 mm.; umbo-ven- 

 tral, 5 "2 ; sectional of one valve, 3 "5 mm. 



Locality. — Cape Jaffa, 130 fathoms, one valve. 



Arcoperna scapha, n. sp. PI. xii., tigs. 1 to 5. 



Shell small, solid, white, oval. Umbos terminal, round, 

 inflated, directed slightly forward, approximate but not in 

 contact. Prodissoconch very distinct, separated definitely 

 from the dissoconch by a fine groove : its earliest third is quite 

 smooth, its later two-thirds engraved with concentric lines, 

 gradually growing more valid. Post-dorsal border nearly 

 straight for 1*7 mm., forming an open rounded angle with 

 the posterior border, which is nearly straight for 11 mm., then 

 sweeps with a slight convexity into the short-curved, circular, 

 ventral border. The anterior dorsal border is very short, al- 

 most entirely under the umbo. The antero-ventral border is 

 slightly convex. The whole dorsum is engraved with crowded, 

 slightly- curved, radial lines, latticed by concentric lines nearly 

 as valid : some of the latter at distant intervals in the early 

 stages of growth are much deeper, so as to form slightly imbri- 

 cating ledges. The inner margin of the shell has fine denti- 

 culations from the end of the post-dorsal border all round 

 and almost to the central extremity of the anterior dorsal 

 border under the umbo ; they do not reach the outer edge of 

 the shell. The straight edge of the prodissoconch is set some- 

 what obliquely to the anterior dorsal border, the central ex- 

 tremity of which extends slightly behind the centre of the 

 edge of the prodissoconch. The post-dorsal line is provided 

 with many (more than 20) close-set, low, vertical teeth on its 

 inner surface. Beneath them is the scar of the ligament for 

 the anterior three-fourths of their extent, somewhat increas- 

 ing in width posteriorly, and having a rounded end. It seems 

 to pass out between the prodissoconch and the anterior dor- 

 sal hinge-line to become external. The anterior dorsal bor- 

 der has a few denticulations and one tooth, and is supported 

 by a sort of thickening which projects slightly into the cavity 

 of the shell and has a rounded central end. Two elongated 

 muscle-scars occupy the middle third of the sides of the shell, 

 not far from the margin, narrowing dorsally ; the posterior is 

 rather the larger. 



