DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



115 



Post. 



Fig. 20. Ledella aberraia. a, dorsal view of shell; b. dorsal, ventral and anterior view of thickened shell; c. lateral view of the hinge plate of a left valve, all 

 from Atlantis II station 247 from the Argentine Basin; d, lateral view of the hinge plate of a right valve, from Chain station 85. Scales = 1mm. 



Distribution. In temperate and tropical basins of the Atlantic at 

 abyssal depths >4000m, rare in the North America Basin. Depth 

 range, 2138-5223m. 



Shell description (Figs 20 & 21) 



Shell small, ovate, relatively high, moderately wide, ornamented 

 with concentric ridges; umbo moderate in size, inwardly turned, 

 anterior to mid-line but less so in juveniles; no lunule or escutcheon; 

 periostracum pale straw colour; postero-dorsal margin slightly con- 

 vex becoming more straight with increasing size, slightly angulate at 

 posterior limit of hinge plate and at posterior margin, postero-ventral 

 margin very slightly sinuous, otherwise ventral margin deeply curved 

 with ventral limit posterior to vertical axis through umbo, anterior 

 margin sharply curved, antero-dorsal margin slightly convex with 

 slight change in slope at anterior limit to hinge plate; shell outline 

 characteristically asymmetrical, shell margin in larger specimens 

 changes direction of growth producing a broad flattened ventral 

 margin; hinge plate broad, up to 6 chevron teeth in anterior series and 

 7 in posterior series, edentulous space between series relatively 

 broad; ligament small, amphidetic, internal part restricted to upper 

 part of hinge plate, external part extremely short situated below 

 umbo. The maximum length of the present specimens is 2.6mm. 



Internal morphology (Figs 22 & 23) 



The adductor muscles are moderately large and oval in shape. The 

 combined siphon is relatively short The dorsal margins of the 

 exhalent part are fused proximally for a short distance and the 

 ventral margins of the inhalent part are not fused but are slightly 

 thickened and probably adhere in living specimens. Internally where 

 inhalent and exhalent parts join, there is a thickened median ridge on 

 each side which together together with the posterior continuation of 

 the gill axes probably act as guides when the faecal pellets are 

 extruded. The siphonal embayment is small and there is a small, 

 slender, tentacle attached to the left side at the inner limit of the 

 embayment. The anterior sense organ is small and is situated ventral 

 to the anterior adductor muscle. 



The palps are small with up to 11 broad ridges. The gills are 

 also small each with up to 11 plates the most posterior of which 

 lies some distance from the siphon. The gills are attached to the 

 posterior limits of the median guides by slender extensions of the 

 gill axis. 



The hind gut is greatly extended and takes a similar but more 

 complex course to that described for L. acinula. Like the latter, the 

 hind gut passes from the right side of the body to the left immedi- 

 ately posterior to the anterior adductor muscle and returns by the 



