DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



75 



strong, extends almost to outer limit of both adductor 

 muscles, anterior hinge plate with 1-2 more teeth than 

 posterior, total number of teeth up to 17 depending on size, 

 distal teeth small and difficult to identify; ligament internal, 

 slightly opisthodetic, 'goblet'-shaped with posterior extension 

 ventral to hinge plate. 



Prodissoconch length: 229 u,m. Maximum recorded shell 

 length: 3.1 mm. 



The shell outline in this species is somewhat variable (Figs. 

 120 & 121). It is the only yoldiellid species indeed species in 

 our collections, with this highly characteristic 'bean'-shape. 

 For a time we misidentified Yoldiella fabula as Y. dissimilis 

 Verrill & Bush 1898. On closer examination we note that Y. 

 dissimilis unlike Y. fabula is nearly equilateral, that the 

 anterior hinge is oblique that the number of hinge teeth is 

 greater and that an oblong prominent tooth-like process at 

 the proximal end of the posterior hinge series is not present. 

 Y. fabula, as all other yoldiellids, has only very small external 

 anterior and posterior componants visible which are derived 

 from secondary fused periostracum. 



The range in length of the specimens in the collection 

 varies from 1.72 mm to 3.08 mm. The following are the 

 overall proportions:- H/L ratio 0.63-0.72; W/L ratio 

 0.41-0.50; and PL/TL ratio 0.40-0.46. Although these ratios 

 vary the overall range is related to growth and to some 

 variation in the populations. 



Internal morphology (Figs. 122 & 123). Ventral to the 

 anterior adductor muscle the middle lobe of the mantle is 

 modified to form a well-developed anterior sense organ. The 

 combined siphon has a single lumen which is open ventrally. 

 The gill axes join laterally indicating an inhalent as well as an 

 exhalent component. A small, single, tentacle is present close 

 to the postero-ventral margin of siphonal embayment. 

 Numerous groups of glandular epithelial mantle cells are 

 found peripheral to the inner siphonal aperture and the 

 feeding aperture and anterior to the latter. The adductor 

 muscles are large and approximately equal in size. The 

 posterior adductor is oval in cross section while the anterior is 

 'crescent'-shaped with 'catch' and 'quick' portions clearly 

 marked. 



The gills are parallel to the dorsal posterior shell margin 

 with up to 17 well-developed plates. The distal gill filaments 

 lie close to the siphon. Moderately large labial palps extend 

 approximately halfway across the body and have up to 20 

 ridges on their inner surface. The foot is well-developed with 

 a narrow neck and a deeply divided sole. At the heel there is 

 a conspicuous median papilla directly posterior to the open- 

 ing of the byssal gland. The byssal gland is well-developed. 

 The pedal musculature is similar to Y. lata. Both cerebral and 

 visceral ganglia are cylindrical and moderately well- 

 developed with a stout connecting commissure. The pedal 

 ganglia are circular and not particularly large. 



The oesophagus, stomach, and style sac are basically 

 similar to those of Y. lata. A long duct from the right 

 digestive diverticula passes dorsally over the hind gut to enter 

 the stomach close to the oesophageal aperture. The duct from 

 one left diverticulum opens slightly more posteriorly on left 

 ventral wall, while the duct from the second left diverticulum 

 opens ventral to the gastric tooth on the left side. The hind 

 gut has a typhlosole and forms a single loop to the right side 

 of the body. 



Initially the gonads develop ventral to the digestive diver- 



ticula and the hind gut loop, but gradully they spread 

 posteriorly and dorsally to the stomach. A female 2.5 mm 

 long, from a July sample had approximately 74 large ova 

 (maximum diameter 180 u-m) while a second female of similar 

 size (2.3 mm) had 62 ova (maximum diameter 160 urn). The 

 gonadial aperture is close to that of the kidney, and anterior 

 to the posterior pedal retractor muscle. The large kidney 

 extends from the postero-dorsal margin to the foot and 

 anteriorly over the viscera to a point just anterior to the 

 lateral pedal retractor muscle. A large pericardial cavity is 

 present. 



Yoldiella veletta (new species) 



Type locality. R.V. Jean Charcot, Biogas VI, Sta. DS87, 

 Bay of Biscay, 31.10.1974, 44°05.2'N, 4°15.7'W, Epibenthic 

 Trawl, 1913 m. 



Type specimen. Holotype: Museum National d'Histoire 

 Naturelle, Paris. 



Material. 



Cruise Sta Depth No Lat Long Gear Date 



(m) 



canaries basin 



Discovery 6701 1934 1 27°45.2'N 14°13.0'W ES 16. 3.68 



CAPE VERDE BASIN 



145 2185 1 10°36.0'N 17°49.0'W ES 6. 2.67 



WEST EUROPEAN BASIN 



Jean Charcot 



Biogas VI DS87 1913 1 44°05.2'N 4°15.7'W ES 31.10.74 



Distributed on the lower slope in the eastern Atlantic from 

 the Bay of Biscay to the Cape Verde Islands. Depth range: 

 1913-2105 m. 



Shell description (Fig. 124). Shell moderately swollen, 



Fig. 120 Yoldiella fabula. Lateral view of a shell from the right 

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

 Sta. BG VI DS 84, West European Basin. (Scale = 1.0 mm). 



