DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



59 



of secretory cells. There is a relatively large single posterior 

 papilla immeditely posterior to the aperture of the byssal 

 gland at the heel of the foot. The gland is large and is 

 surrounded by secretory cells. The hind gut is complex with 

 twinned loops on each side of the body, with two recurved 

 sections immediately anterior to the stomach. The main 

 lateral loops of the hind gut cross from one side of the body to 

 the other dorsally and immediately posterior to the resilium. 

 All the specimens examined had well-developed gonads 

 which makes it difficult to determine the exact course of the 

 gut, but from sections we are confident that it has a similar 

 design to that described for Yoldiella subcircularis . One 

 sectioned female (1.63 mm total length) from Sta. 299 has 18 

 large ova present (maximum observed dimension 114 |xm). 



Despite the similarity of the internal morphology to that of 

 Yoldiella subcircularis the shapes of the shell, hinge and 

 ligament are totally different in the two species (p. 55). As we 

 point out elsewhere the various types of hind gut morphology 

 do not necessarily correlate to a particular characteristic suite 

 of shell characters. All that can be said with certainty is that 

 the hindgut of Yoldiella biguttata has configuration so far 

 found only in the Yoldiellidae and only in species from the 

 abyss and abyssal rise. 



Yoldiella ovata (new species) 



Type locality. R.V. Knorr Cruise 25, Sta. 300, Guyana 

 Basin, 29.2.1972, 8°14.2'N, 55°53.5'W, Anchor Dredge, 

 2524-2542 m. 



Type specimen. Holotype: BM(NH) 1992035. Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise Sta Depth No Lat Long Gear Date 



(m) 



GUYANA BASIN 



Knorr 25 300 2524- 208 8°14.2'N 55°53.5'W AD 29.2.72 



2542 

 301 2487- 324 8°12.4'N 55°50.2'W ES 29.2.72 



2500 

 303 2842- 13 8°28.8'N 56°04.5'W ES 1 .3.72 



2853 



Restricted to the abyssal rise of the Guyana Basin. Depth 

 range: 2487-2853 m. 



Shell description (Figs. 91 & 92). Shell small, ovate, 

 moderately inflated, inequilateral, transparent, dorso- 

 lateral^ smooth, ventrally, with very fine concentric lines 

 forming ridges; umbos slightly anterior of midline, moder- 

 ately inflated, orthogyrate; dorsal margin slightly convex, 

 antero-dorsal and postero-dorsal margins with similar curva- 



Fig. 91 Yoldiella ovata. Lateral views of a shell from the left side 

 and dorsally and an inner view of a left valve to show detail of the 

 hinge-plate. Specimens taken from Sta. 300, Guyana Basin. 

 (Scale = 1.0 mm). 



Fig. 92 Yoldiella ovata. Outlines of shells from the right side to 

 show change in shape with growth of specimens from Sta. 300, 

 Guyana Basin. (Scale = 1.0 mm). 



