DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



23 



and at least three times as large as the posterior. The foot is 

 large, anteriorly directed and with a large byssal gland. The 

 cerebral and visceral ganglia are relatively large, club-shaped 

 with a stout commissure. The pedal ganglia are also large but 

 more rounded. The labial palps are well-developed and extend 

 across approximately half of the body and have up to 14 broad 

 internal ridges present on their inner face. The palp proboscides 

 are moderately long. The gills are small with up to 18 alternating 

 filaments. 



The stomach is large and laterally distended. The proximal 

 part of the hind gut penetrates deep into the foot, ventral to 

 the pedal ganglia. It forms a single loop on the right side of 

 the body. 



In external appearance the shells of the species can be 

 easily confused with Yoldiella artipica (p. 25) although Y. 

 hanna is slightly more inflated and the height/length ratio is 

 slightly greater (Fig. 20). Nevertheless, hinge and teeth differ 

 markedly, as does the hind gut configuration. 



Yoldiella capensis (new species) 



Type locality. R.V. Atlantis II, Cruise 42, Sta. 189, Cape 

 Basin, 15.5.1968, 23°00'S, 12°45'E, Epibenthic Trawl, 

 1007-1014 m. 



Type specimen. Holotype BM(NH) 1992026, Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise 



Sta 



Depth No Lat 

 (m) 



Long 



Gear Date 



CAPE BASIN 



Atlantis II 188 619- 70 23°00.0'S 12°58.0'E ES 16.5.68 

 42 622 



189 1007- 918 23°00.0'S 12°45.0'E ES 15.5.68 

 1014 



190 974- 15 23°05.0'S 12°45.0'E AD 17.5.68 

 979 



Only found at slope depths of the Cape Basin. Depth range: 

 619-1014 m. 



Shell description (Figs. 21 & 22). Shell, subovate, moder- 

 ately inflated, inequilateral, posteriorly somewhat wedge- 

 shaped, smooth with a few fine concentric lines, periostracum 

 pale straw-coloured; umbos slightly raised, inwardly directed; 

 antero-dorsal margin convex, slopes steeply from umbo to 

 anterior margin, dorsally posterior margin produced into 

 rounded point, postero-dorsal margin long, varying from 

 slightly convex in smaller specimens (usually) to slightly 

 concave, slopes gradually to posterior margin, hinge plate 

 strong, relatively long, plates approach shell margin below 

 umbo, anterior plate arched with up to 9 erect chevron teeth, 

 posterior plate extends to anterior margin of adductor, 

 straighter than anterior, with up to 11 teeth and does not 

 extend beyond posterior margin of adductor; ligament 

 amphidetic, bilobed in lateral view, extends below hinge line. 



Prodissoconch length: 170 |xm. Maximum recorded shell 

 length: 8.0 mm. 



With increasing size this species becomes more posteriorly 

 elongate and the distal posterior dorsal limit more pointed 

 (Figs. 21 & 22). Furthermore the posterior dorsal margin 

 becomes less curved with increasing size, so much so that 



Fig. 21 Yoldiella capensis. Lateral view from the left side of the 

 largest shell taken from Sta. 189 Cape Basin and hinge detail of 

 right valve of the same specimen. (Scale = 1.0 mm). 



Fig. 22 Yoldiella capensis. Lateral views of shells from the right 

 side to show change in shape with growth. Specimens from Sta. 

 188 Cape Basin. (Scale = 1.0 mm). 



without a size series it would be difficult to equate small 

 specimens with large. Other shell ratios (H/L and W/L) 

 remain more or less constant as length increases (Fig. 23). 



Internal morphology (Fig. 24). Specialization of the 

 mantle includes a well-developed anterior sense organ and 

 combined exhalent and inhalent siphons. The latter are 

 well-developed with thick muscular walls. A siphonal tentacle 

 lies to the left of the siphons. There is a feeding aperture 

 ventral to the siphons which is much folded in the contracted 

 state. The adductor muscles are relatively small. The poste- 

 rior muscle is narrow and elongate, while the anterior is 2 to 3 

 times larger and crescent-shaped. 



The gills are well-developed with up to 20 plates. The labial 

 palps are moderate in size. They extend between 1/4 - 1/3 

 distance across the body and have long and slender palp 

 proboscides. The nervous system is well developed with large 

 club-shaped visceral and cerebral ganglia and massive com- 

 missures which link them. Large, round, pedal ganglia each 

 have a large statocyst full of refractile granules dorsal to 



