DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



51 



those described by Stasek (1965) for Yoldia ensifera. 



The ciliation on the frontal surface of the gill plates is 

 axially directed, there those on the posterior 2/3rds of the axis 

 move anteriorly and those on the anterior l/3rd move poste- 

 riorly. Particles thus accumulate at a point which is close to 

 where the palp proboscides join the dorsal palp surface and 

 they are accepted onto the palp. The palp ridges sort in the 

 manner described by Stasek (1965). Rejected particles arrive 

 at the posterior-ventral margin of the palp. A main rejection 

 tract at the mantle edge to the inside of the muscular fold 

 carries these particles to a point of collection at the ventral 

 edge of the feeding aperture. Particles on the body eventually 

 pass to the dorsal margin of the palps and join with those 

 from the gills. 



Hydroids were present on the shell margins of a number of 

 specimens. 



Yoldiella perplexa (new species) 



Type locality. R.V. Knorr Sta. 299, Guyana Basin, 

 29.2.1972, 7°55.1'N, 54°42.0'W, Epibenthic Trawl, 

 1912-2076 m. 



Type specimen. Holotype BM(NH) 1992031, Paratypes: in 

 collection held by J. A. Allen. 



Fig. 76 Yoldiella curia. Lateral views from the right and left sides 

 of the internal morphology of a specimen from Sta. BG VI DS 71 

 West European Basin. (Scale = 1.0 mm). For identification of the 

 parts see Fig. 34. 



Fig. 77 Yoldiella perplexa. Lateral view of shell from the right 

 side, lateral internal view of a right valve to show detail of 

 hinge-plate and a dorsal view of a shell. Specimens from Sta. 299 

 Guyana Basin. (Scale = 1.0 mm). 



Material. 



Cruise Sta Depth No Lat Long Gear Date 



(m) 



GUYANA BASIN 



Knorr 25 293 2456- 22 8°58.0'N 54°04.3"W ES 27.2.72 

 1518 

 299 1942- 82 7°55.1'N 54°42.0'W ES 29.2.72 

 2076 



Recorded from the lower slope of the Guyana Basin. Depth 

 range: 1456-2076 m. 



Shell description (Fig. 77). Shell small, ovate, moderately 

 inflated, inequilateral, smooth with some fine irregularly 

 spaced concentric lines at ventral margin, periostracum pale 

 yellow; umbos moderately developed, slightly raised, 

 inwardly directed anterior to midline; proximal dorsal margin 

 straight in small specimens, slightly convex in larger, antero- 

 dorsal margin curves gradually to wide anterior margin, 

 dorsal part slightly more convex than ventral, ventral margin 

 deeply convex, postero-dorsal margin slopes gradually to 

 steeply rounded posterior margin, postero-ventral margin 

 relatively straight and in some specimens very slightly sinous; 

 hinge plate moderately broad, not narrow below umbo, hinge 

 teeth strong, few in number, 6 anterior and 6 posterior in 

 individual 2.6 mm; ligament amphidetic, moderate in size, 

 not extending to ventral edge of hinge plate, round in lateral 



