40 



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



directed; proximal antero-dorsal margin horizontal for very 

 short distance before curving steeply to anterior margin, 

 proximal postero-dorsal margin horizontal or slightly convex, 

 distally slightly angled opposite posterior limit of hinge, 

 postero-ventral margin slightly sinous giving a relatively 

 narrow rounded medial tip to posterior margin, ventral 

 margin smooth convex curve. Hinge plate short, moderately 

 narrow with up to 9 small chevron-shaped teeth on each 

 series, one additional tooth on posterior plate. Ligament 

 large, amphidetic, bilobed in lateral view, with short, narrow, 

 secondary external extensions anterior and posterior to 

 umbo. 



Prodissoconch length: c. 200 |xm. Maximum recorded shell 

 length: 4.0 mm. 



Shell measurements (mm) & ratios are as follows:- 



Length Height Width H/L W/L PL/TL Teeth(A/P) 



1.89 



1.31 



0.70 



0.70 



0.37 



0.51 



- 



2.58 



1.76 



0.98 



0.68 



0.38 



0.53 



- 



2.62 



1.80 



0.94 



0.69 



0.36 



0.56 



- 



2.81 



1.89 



1.09 



0.67 



0.39 



0.55 



- 



3.05 



2.05 



1.23 



0.67 



0.40 



0.54 





3.44 



2.21 



0.64 



- 



- 



0.57 



8/9 



3.55 



2.38 



1.44 



0.67 



0.40 



0.55 



- 



3.61 



2.46 



0.68 



- 



- 



0.53 



7/8 



3.81 



2.67 



1.52 



0.70 



0.40 



0.56 



- 



3.98 



2.62 



1.58 



0.66 



0.40 



0.59 



- 



Internal morphology (Fig. 57). The internal morphology 

 is very similar to that described for Y. frigida. The siphons are 

 combined and there is a siphonal tentacle to the left. The 

 anterior sense organ is well-developed. The adductor muscles 

 are relatively large, the posterior being more round and 

 slightly smaller than the anterior muscle. The foot is also 

 large with a moderately-sized byssal gland. The gills are well 

 developed and suspended more ventral within the mantle 

 cavity than other species. There are approximately 15 alter- 

 nating gill filaments. The labial palps extend across about half 

 the body and have 12-16 internal ridges and well-developed 

 palp proboscides. The visceral and cerebral ganglia are 

 relatively large. The viscera are also more ventral in position 

 than in most other Yoldiella species. The pedal ganglia are 

 large and elongate. The hind gut forms a single loop on the 

 right side of the body. 



Yoldiella externa (new species) 



Type locality. R.V. Atlantis II, Cruise 60, Sta. 245, 

 Argentine Basin, 14.3.1971, 36o55.7'S, 53o01.4'W, 

 Epibenthic Trawl, 2707 m. 



Type specimen. Holotype: BM(NH) 1992036, Paratypes: in 

 collection held by J.A. Allen. 



Material. 









Cruise Sta Depth No 

 (m) 



Lat 



Long 



Gear Date 



argentine basin 



Atlantis II 245 2707 26 

 60 



36°55.7'S 



53°01.4'W 



ES 14.3.71 



Fig. 58 Yoldiella extensa. Lateral view of a shell from the right side 

 and detail of the hinge -plate of a right valve. Specimens from Sta. 

 245 Argentine Basin. (Scale = 1.0 mm). 



Shell description (Figs. 58 & 59). Shell oblong-ovate, 

 moderately robust, moderately inflated, nearly equilateral, 

 very fine concentric lines, periostracum pale yellow; umbo 

 inflated, raised slightly, directed posteriorly; dorsal margin 

 slightly convex, antero-dorsal margin gradually slopes to 

 broad rounded anterior margin, ventral margin long, curva- 

 ture similar to that of dorsal margin, posterior margin slightly 

 extended with faint suggestion of postero-ventral sinus; liga- 

 ment amphidetic, moderate size, elongate, but slightly asym- 

 metric in lateral view with posterior part slightly longer and 

 wider than anterior, slight external extension on either side of 

 umbo; hinge plate narrow, relatively short, with small eden- 



Only found at the one Station at abyssal depth in Argentine 

 Basin. Depth range: 2707 m. 



Fig. 59 Yoldiella extensa. Outlines of shells from the right side to 

 show change in shape with growth (see also Fig. 58). Specimens 

 from Sta. 245 Argentine Basin. (Scale = 1.0 mm). 



