22 



J.A. ALLEN, H.L. SANDERS AND F. HANNAH 



diameter of 83 |xm (probably less than half their eventual 

 maximum size). 



Yoldiella hanna (new species) 



Type locality. R.V. Atlantis II Cruise 42, Sta. 186, Cape 

 Basin, 15.5.1968, 22°57'S, 13°05'E, Epibenthic Trawl, 

 439-481 m. 



Type specimen. Holotype BM(NH) 1992022, Paratypes: in 

 collection held by J.A. Allen. 



Material. 



Cruise Sta Depth No Lat Long Gear Date 



(m) 



CAPE BASIN 



Atlantis II 186 439- 21 22°57.0'S 13°05.0'E ES 16.5.68 

 42 481 



188 619- 7 23°00.0'S 12°58.0'E ES 16.5.68 

 622 



The species is restricted to the upper slope in the Cape Basin. 

 Depth range: 439-622 metres. 



Shell description (Fig. 18). Shell small, ovate, relatively 

 inflated, inequilateral (posterior umbonal length 52-57% of the 

 total length), sculpture, fine concentric lines at ventral margin, 

 pale straw-coloured periostracum; umbos slightly raised, 

 inwardly directed; dorsal margin slightly convex, antero-dorsal 



Fig. 19 Yoldiella hanna. Lateral view from right side of the 



internal morphology. Specimen from Sta. 186 Cape Basin. (Scale 

 = 1.0 mm). For identification of parts see Fig. 5. 



margin slopes gradually and smoothly to anterior margin, dorsal 

 part of which more convex, ventral margin long, smooth, curve, 

 postero-dorsal margin almost straight maybe slightly concave, 

 slopes gently to blunt-angled supramedial posterior margin; 

 hinge plate very strong, wide, except centrally under umbo; 10 

 strong chevron-shaped teeth on each side of ligament in largest 

 specimen; ligament amphidetic, goblet-shaped, extends below 

 margin of hinge plate. 



Prodissoconch length: 200 u.m. Maximum recorded shell 

 length: 3.2 mm. 



Internal morphology (Fig. 19). Well-developed combined 

 siphons are present with a well-developed feeding aperture 

 ventral to them. The siphonal tentacle lies to the left. The 

 posterior adductor muscle is very small and elongate oval in 

 cross-section. The anterior adductor muscle is crescent-shaped 



60-, 



40 

 80-1 



H/L 



60-- 



60- 

 50-- 



W/L 



o o 



• 'At •• • • 



PL/TL 



Length (mm) 



Fig. 18 Yoldiella hanna. Lateral views from the right side of three 

 shells and an internal view of a right valve to show detail of the 

 hinge plate. Specimens from Sta. 186 Cape Basin. (Scale = 1.0 

 mm). 



Fig. 20 Yoldiella spp. Comparison of the shell proportions of Y. 

 hanna (closed circles) and Y. artipica (open circles). Height H/L, 

 width W/L and postero-umbonal length PL/TL to length against 

 length. 



