DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



57 



and well-developed. The labial palps are large, with up to 32 

 closely spaced ridges on each inner face. In some of the 

 largest specimens (> 3.3 mm) the anterior part of the palps 

 are frequently folded in on themselves. The palp proboscides 

 are relatively small and slender. The foot is conspicuous, 

 being long and thin, with a much extended narrow neck and a 

 long divided papillate sole. Gland cells are present along the 

 ventral part of the sole and surrounding the aperture of a 

 large byssal gland. Pedal retractor muscles are less conspicu- 

 ous in this species than in others described here. The central 

 haemocoele of the foot is extensive. The cerebral and visceral 

 ganglia are relatively small, circular in transverse vertical 

 cross-section. The visceral ganglia are some distance anterior 

 to the posterior adductor muscle. The pedal ganglia lie at the 

 dorsal limit of the extended neck of the foot close to its 

 junction with the body, they are large and elongate with large 

 associated statocysts dorsal to them. 



The mouth is displaced a short distance posteriorly from 

 the anterior adductor muscle. The stomach and style sac are 

 moderately large. Two broad sorting ridges can be seen on 

 the right anterior wall of the stomach. A prominent gastric 

 tooth is present on the left dorsal wall and posterior to it the 

 gastric shield extends over the left and most of the right walls 

 of the stomach. The major typhlosole extends along the right 

 side of the stomach. The digestive diverticula lie either side 

 and anterior to the stomach. The duct of the right digestive 

 diverticulum curves dorsally over the hind gut and enters the 

 stomach on the right anterior dorsal wall. A duct from one 

 left diverticulum enters the stomach far anterior on the left 

 dorsal side, and immediately posterior to it, a short duct from 

 a second left diverticulum enters the stomach, ventral to the 

 gastric tooth. Material similar to that in the stomach was 

 present in the lumen of the latter diverticulum, but this could 

 possibly have resulted from tissue contraction following fixa- 

 tion. The style sac is relatively large and extends into the 

 dorsal part of the foot. The hind gut has a shallow typhlosole 

 along its entire length, the impression of which can be seen on 

 extruded faecal pellets. The hind gut is arranged in two loops 

 on each side of the body. These are usually visible through 



Fig. 88 Yoldiella subcircularis. Dorsal view of the hind gut of a 

 specimen from Sta. 80 North America Basin and a diagrammatic 

 view of the course of the hind gut as seen from the left side. 

 (Scale =1.0 mm). 



the shell antero-dorsally. There is also a double vertical 

 extension of the hind gut deep into the foot. Thus, design 

 advantage is taken of the exceptional length of the neck of the 

 foot to accommodate a significant part of the greatly 

 extended hind gut. 



The sexes are separate. Sectioned specimens larger than 

 2.8 mm had maturing gonads. The number of ova ranged 

 from 37 (2.80 mm) to approximately 140 (2.91 mm) with a 

 maximum ovum diameter of 132 \x.m and 156 um respectively. 

 The gonads overlie the lateral and dorsal sides of the viscera. 

 Mature females were present in February, June, August and 

 October samples from the West European Basin. In two 

 specimens from (Sta. DS76, Sta. DS79 respectively both 2.91 

 mm total length), eggs were partially shed into the mantle 

 cavity and had a maximum dimension of 156 u.m. Neverthe- 

 less, there is no evidence to show that eggs are retained and 

 incubated within the mantle cavity. 



Although the kidney is relatively small, it extends forward 

 on either side of the stomach for a short distance. 



Yoldiella biguttata (new species) 



Type locality. R.V. Knorr Cruise 25, Sta. 299,Guyana 

 Basin, 29.2.1972, 7°55.1'N, 55°42.0'W, Epibenthic Trawl, 

 1942-2076 m. 



Type specimen. Holotype: BM(NH) 1992029. Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise Sta Depth No Lat Long 



(m) 



Gear Date 



BRAZIL BASIN 



Atlantis II 167 943- 5 

 31 1007 



ARGENTINE BASIN 

 Atlantis II 245 2707 2 

 60 



7°58.0'S 34°17.0'W ES 20.2.67 

 - 7°50.0'S 34°17.0'W 



36°55.7'S 53°01.4'W ES 14.3.71 



8°58.0'N 54°04.3'W ES 27.2.72 



8°04.2'N 54°21.3'W ES 28.2.72 



7°55.1'N 55°42.0'W ES 29.2.72 



8°12.4'N 55°50.2'W ES 29.2.72 



GUYANA BASIN 



Knorr 25 293 1456- 13 



1518 

 295 1000- 2 



1022 

 299 1942- 74 



2076 

 301 2487- 44 



2500 

 303 2849- 4 8°28.8'N 56°04.5'W ES 1. 3.72 



1853 



Y. biguttata is distributed off the coast of eastern South 

 America at mid slope to abyssal depths in the Argentine, 

 Brazil and Guyana Basins. Depth range: 943-2853 m. 



Shell description (Fig. 89). Shell small, inflated, equilat- 

 eral, fine concentric striae; umbos posterior to midline, 

 slightly raised, moderately large, inwardly directed; dorsal 

 margin raised, sharp-edged, particularly so anterior to umbo, 

 dorsal margin close to umbo straight, proximal antero-dorsal 

 margin curves to broadly rounded anterior margin, the limit 

 of which is slightly ventral to the midline, ventral margin 

 shallow curve, in some specimens almost straight centrally, 

 postero-ventral margin sinuous giving a characteristic oblique 



