DEEP-SEA PROTOBRANCHIA (BIVALVIA) 

 The type specimen has been examined by us. 



Distribution. Found predominantly in tropical and subtropical 

 latitudes, and southern temperate latitudes in the eastern Atlantic, at 

 mid-slope to abyssal depths ranging from 943m to 4632m. 



Many protobranch species with robust shells having subrostrate 

 or ovate outlines and with concentric ridges and external liga- 

 ments, in the past have been referred to the genera Malletiu, 

 Neilonella oxTindaria (e.g. Dall, 1890). Ledella acinula is a case 

 in point. Having addressed this problem in earlier papers (Sand- 

 ers and Allen. 1977, 1985), and the present, we have been able to 

 define more rigorously the families Tindariidae, Neilonellidae 

 and Malletiidae. 



Dall ( 1 890) who described large specimens of L. acinula referred 

 them to the subgenus Tindaria. Large specimens do show some 

 resemblance to neilonellids and tindariids, but had Dall seen the 

 shells of smaller specimens (Fig. 1 8), he would have been unlikely to 

 have made the error. 



In our studies on Tindaria (Sanders and Allen, 1977) we re- 

 examined and briefly redescribed L. acinula, and recognized that 

 there was a problem in identification but deferred final judgment 

 until we had made further comparative studies. Now that the 

 Ledellinae have been reported upon (Allen and Hannah, 1989), the 

 taxonomic relationship of this species is clear. 



ThatL. acinula is siphonate clearly distinguishes it from members 

 of the family Tindariidae. Although similar to the neilonellids in 

 having an external amphidetic ligament, it differs in having a small 

 but well-defined internal ligament and in being semi-rostrate with a 

 slightly sinuous postero-ventral margin. A further significant differ- 

 ence is the form of the hind gut and the course that it takes within the 

 body (Sanders and Allen, 1977). In L. acinula the hind gut is not 

 particularly wide in diameter and is not restricted to the right side of 

 the body (see below) having a configuration only known to occur in 

 species of the subfamily Ledellinae e.g.Ledella galatheae Knudsen 



113 



1970, L. oxira (Dall 1927), L. acuminata (Jeffreys 1870)(Allen and 

 Hannah, 1989). 



Shell description (Figs 17 & 18) 



Shell robust, posteriorly angulate, ornamented with concentric ridges 

 particularly well-defined on ventral part of shell, straw-coloured 

 periostracum; umbos relatively low in profile, inward facing, anterior 

 to midline; posterior rostral region characteristically broad and blunt 

 when seen in dorsal view; postero-dorsal shell margin almost straight, 

 angulate at posterior limit of hinge plate - particularly so in smaller 

 specimens, barely so in larger, posterior margin sharply rounded, 

 ventral margin deeply curved, postero-ventral margin slightly sinu- 

 ous, particularly in smaller specimens, antero-dorsal, anterior and 

 antero-ventral margins form a smooth curve; posterior and anterior 

 limits of shell at or slightly ventral to mid horizontal axis, ventral limit 

 of shell posterior to vertical axis through umbo; hinge plate elongate, 

 broad, anterior and posterior tooth series separated by relatively long 

 edentulous space, chevron-shaped teeth acutely angled, up to 10 

 anterior and 12 posterior teeth depending on size of specimen; 

 ligament amphidetic, external parts short, internal resilium small, 

 rounded, occupying upper central part of hinge plate below umbo.The 

 maximum total length of the present specimens is 6.0mm. 



The shape of the shell changes significantly with growth (Figs 18 

 & 19). While the ratio of height to length remains more or less the 

 same, the postero-umbonal length increases from 50% to 60% of the 

 total length of the shell. With increasing size the postero-dorsal 

 margin also becomes less angulate at the posterior limit of the hinge, 

 also the postero-ventral margin becomes less sinuous, at most being 

 somewhat flattened. 



Internal morphology 



This has been described and illustrated by us in our earlier studies on 

 the family Tindariidae (Sanders and Allen, 1977). Only essential 

 features relating to the taxonomy need be mentioned. 



Combined siphons are present and there is a well-developed 



Fig. 17. Ledella acinula. a & b. internal and external views of a left valve, from the type locality Albatross station 2754. USNM 95438; c, internal view 

 of left valve of specimen, from Atlantis II station 167 from the Brazil Basin; d & e, lateral view of left side and dorsal view of a shell, from Knorr station 

 299 from the Guyana Basin. Scale = 1mm. 



