DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



71 



Fig. 113 Yoldiella ella. Lateral view of a shell from the right side 

 and a left valve to show detail of the hinge-plate. Specimens from 

 Sta. INCAL OS 06. (Scale = 1.0 mm). 



est shell height posterior to umbo; dorsal margin slightly 

 convex, antero-dorsal anterior and ventral margins in a 

 smooth continuous curve, postero-ventral margin rounded 

 posteriorly, posterior margin in small specimens may have 

 very slight, blunt, angulation, posterior limit slightly dorsal to 

 horizontal midline, postero-dorsal margin curves gently from 

 umbo to distal edge to hinge plate then slopes more acutely to 

 posterior margin; hinge plate, long, characteristically angular 

 below umbo, moderately broad proximally, narrow ventral to 

 umbo, anterior and posterior ventral margins of hinge plate 

 more or less straight, teeth strong, well-developed, equal 

 number in anterior and posterior plates, up to 9 in specimen 

 4.2 mm, ligament amphidetic, moderate in size, rectangular 

 or slightly 'goblet'-shaped, short, wide, posterior external 

 extension and long slender anterior external extension of 

 fused periostracum. 



Prodissoconch length: 166 u.m. Maximum recorded shell 

 length: 4.2 mm. 



There is little change in the width/length or height/length 

 ratios with increasing size, however, posterior umbonal 

 i length increases slightly and the posterior margin becomes 

 more smoothly curved. 



Internal morphology (Fig. 117). The anterior sense 

 organ is well-developed. Posterior mantle fusion is minimal, 

 ;limited to a fine bridge of tissue between the opposing inner 

 mantle lobes forming a short exhalent siphon. The gill axis is 

 attached laterally on either side of the bridge tissue. Ventral 

 to it the inhalent siphon is reduced to a pair of unfused 

 ithickened pads of tissue. The feeding aperture is poorly 

 developed with a few gland cells present. Immediately ante- 

 rior to the feeding aperture, the inner mantle fold is enlarged 



Fig. 114 Yoldiella ella. Outlines of shells from the right side to 

 show variation in shape with growth of specimens from Sta. BG 

 VI DS 76, West European Basin. (Scale = 1.0 mm). 



slightly and more obviously ciliated and may possibly indicate 

 a temporary point of adhesion in the living specimen. The 

 siphonal tentacle is usually to the left ventral side of the 

 shallow mantle embayment. The adductor muscles are rela- 

 tively small, approximately equal in size and more or less oval 

 in shape. There are up to 16 gill plates and the gills are 

 attached far posterior on the body wall. The gill axis is 

 well-supplied with muscle fibres. The labial palps, like the 

 gills, are relatively small with up to 14 ridges on their inner 

 faces with the result that in preserved specimens, there is a 

 marked separation of gill and palp which is unlikely to be true 

 in life. The palp proboscides are relatively long and broad. 

 The foot is of moderate size with a relatively wide neck and 

 an elongate, deeply divided sole. There is a large byssal gland 

 in the heel with a concentration of cilia around its opening. 

 The heel has a terminal papilla. The cerebral and visceral 

 ganglia are relatively small and 'club'-shaped in lateral view. 

 The pedal ganglia is larger and slightly elongate. 



The oesophagus has a wide opening into a large stomach. 

 The combined style sac and mid gut extend into the dorsal 

 half of the foot. The digestive duct from the right diverticu- 

 lum skirts dorsal to the hind gut loops to open into the 

 anterior wall of the stomach. On the left and close to the 

 oesophageal aperture is a relatively wide digestive duct from 



