DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



107 



SP — 



Fig. 8. Neilonella salicensis. Detail of internal morphology; a, lateral 

 view from the right side of the stomach and style sac; b, internal view of 

 a dissected gastric shield; c, diagrammatic enlargement of the siphonal 

 region to show the relationship of the parts. See abbreviations to the 

 text-figures p. 102. Scales = 0.5mm (A); 0. 1 mm (B). 



protobranchs (Purchon, 1 956), is one third lined with a gastric shield 

 dorsally and to the left (Fig.8). The unlined right wall bears very few 

 sorting ridges. The style sac is flask-shaped rather than the more 

 usual conical outline of other protobranchs. The combined mid gut 

 is guarded by minor and major typhlosoles. The hind gut penetrates 

 the foot to a point ventral to the pedal ganglia before taking a course 

 posterior to the stomach and thence to form a loop to the right side 

 of the body. This single loop extends anteriorly as far as the anterior 

 adductor muscle and where it then takes a dorsal course along the 

 margin of the body to the anus. The hind gut has a particularly wide 

 diameter compared with most other deep-sea protobranchs and has 

 a well-marked typhlosole present along its entire length. The diges- 

 tive gland which lies anterior and dorso-lateral to the stomach has 

 three sections each connecting with the stomach via a duct. The 

 apertures of the ducts are ventral, and left latero-ventral to the 

 oesophageal aperture. 



The nervous system is of a typical protobranch design with 

 elongate pairs of cerebral and visceral ganglia and large ovoid pedal 

 ganglia. These are connected by very stout cerebro-visceral and 

 cerebro-pedal commissures. A statocyst lies dorsal each pedal gan- 



glion. From visceral ganglia three main nerves pass to the gill, hind 

 gut and mantle/siphon, and from the cerebral ganglia nerves pass to 

 the palps and mantle. 



The sexes are separate, specimens greater than 4mm total length 

 have gonads. The gonad first develops at the periphery of the visceral 

 mass and as it matures it spreads across the lateral surface of the 

 viscera. On the right side of the body, apart from a small portion 

 postero-dorsal to the loop of the hind gut the gonad does not extend 

 beyond the confines of the of the loop. While different samples show 

 different degrees of maturity, the specimens of each sample appear 

 to be maturing synchronously. The ratio of males to females is 

 approximately 50:50. 



Neilonella hampsoni new species 



Type specimen. Holotype BMNH 1995049; paratype BMNH 

 1995050. 



Type locality. Atlantis II cruise 31, station 155, Sierra Leone 

 Basin, 00°03.0'S 27°48.0'W, 3730-3783m. 



Material: 



Cruise Sta Depth No Lat Long Date Gear 



(m) 



SIERRA LEONE BASIN 



Atlantis 11 155 3730-3 00'03.0'S 27'48.0'W 14.2.67 ES 



3 1 3783 



J.Charcot DS03 5150 I 1059.0'N 45T5.0'W 16.1 1.77 DS 



Vema 



Distribution. An abyssal species, to date only found in the Sierra 

 Leone Basin at depths from 3730m to 5 1 50m. 



Shell description (Figs 9 & 10) 



Shell robust, oval, wide, ornamented with concentric ridges, straw- 

 coloured periostracum: umbo prominant, anterior to midline 

 (postumbonal length c.60% of total length in large specimens), 

 inward facing; lunule and escutcheon, both well-defined; postero- 

 dorsal margin in lateral view almost straight, slopes relatively 

 steeply from umbo to proximal limit of hinge, rounded angle to 

 posterior margin, anterior and ventral margins form a smooth curve, 

 distally antero-dorsal margin slightly concave then slopes steeply in 

 smooth curve to anterior margin; anterior limit of shell coincides 



Fig. 9. Neilonella hampsoni. Lateral view of the right valve of the 

 holotype, from Atlantis II station 155 from the Sierra Leone Basin. 

 Scale = 1mm. 



