DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



19 



heavily ciliated throughout. The adductor muscles are rela- 

 tively large and unequal in size. The posterior muscle is oval 

 in outline and is approximately 1/2 to 1/3 the size of the 

 anterior muscle. 



The gills are well-developed with up to 16 gill filaments 

 depending on the size of the individual. The number of ridges 

 on the inner surface of the labial palp varies between 12-17. 

 The palps extend over approximately half the body width and 

 have long, moderately thick palp proboscides. The visceral 

 and cerebral ganglia are elongate and each large pedal 

 ganglia has a large statocyst dorsal to it. The foot is large and 

 directed anteriorly, with well-developed retractor muscles. 

 There is a moderately large byssal gland in the heel. The 

 oesphagus is wide and opens into a voluminous stomach. The 

 style sac penetrates into the lower portion of the foot. The 

 hind gut forms a single loop on the right side of the body and 

 has a typhlosole along its entire length. The digestive diver- 

 ticula, as in other species, are situated one to the right and 

 two to the left of the stomach. The ducts to the left hand 

 diverticula enter the stomach close together, ventral to the 

 gastric shield one posterior to the other. The kidney is small. 

 Gonads overlie the lateral and dorsal sides of the viscera and 

 the sexes are separate. 



Y. obesa s.s. and Y. lucida differ as follows:- (1) The 

 ventral margin of Y. obesa is slightly deeper and more 

 rounded. (2) The anterior adductor of Y. obesa is relatively 

 smaller when compared with a specimen of Y. lucida the 

 same size. (3) The internal ligament of Y. obesa is smaller, 

 shorter and more rounded. (4) The umbo is more anterior in 

 Y. obesa and slightly larger and more prominent. (5) The 

 posterior margin is more rostrate and tapered in Y. obesa and 



Fig. 11 Yoldiella obesa incala. Lateral view of shells from the right 

 side to show change in shape with increasing size, and detail of 

 the hinge plate of a right valve. Specimens from Incal Sta. DS02. 

 (Scales =1.0 mm). 



less truncate. (6) The posterior section of the loop of the hind 

 gut is less curved and almost vertical in Y. obesa. 



Yoldiella obesa incala (new subspecies) 



Type locality. R.V. Jean Charcot, Cruise INCAL, Sta. 

 DS01, East of Rockall Island, 15.7.1976, 57°59'N, 10°40'W, 

 Epibenthic Trawl, 2091 m. 



Type specimen. Holotype: Museum National d'Histoire 

 Naturelle, Paris, Paratypes: in collection held by J. A. Allen. 



Material. 



Cruise 



Sta Depth 

 (m) 



No 



Lat 



Long 



Gear Date 



WEST EUROPEAN BASIN 











Jean Charcot DS25 2096 



3 



44°08.2'N 



4°15.7'W 



DS 



1.11.71 



(Polygas) 



DS26 2076 



3 



44°08.2'N 



4°15.0'W 



DS 



1.11.71 



Jean Charcot DS51 2430 



1 



44°11.3'N 



4°15.4'W 



DS 



12. 2.74 



(Biogas IV) 















(Biogas V) 



CP07 2170 



4 



44°09.8'N 



4°16.4"W 



CP 



21. 6.74 



(Biogas VI) 



DS80 4120 



2 



46°29.5'N 



10°29.5'W 



DS 



27.10.74 





DS86 1950 



7 



44°04.8'N 



4°18.7'W 



DS 



31.10.74 





DS87 1913 



5 



44°05.2'N 



4°19.4'W 



DS 



1.11.74 



(Incal) 



DS01 2091 



518 



57°59.0'N 



10°40.0'W 



DS 



15. 7.76 





DS02 2081 



452 



57°58.0'N 



10°49.0'W 



DS 



16. 7.76 





CP01 2068- 



35 



57°57.0'N 



10°43.0'W 



CP 



16. 7.76 





2040 















CP02 2091 



3 



57°58.0'N 



10°43.0'W 



CP 



16. 7.76 



Restricted to the West European Basin at lower slope depths. 

 Depth range: 1913-2170 mm. 



Shell description (Figs. 11 & 12). Y. obesa incala is similar 

 to Y. obesa s.s. and requires little description other than to 

 identify points of differences with the North American sub- 

 species. 



50 



W/L 



30 

 80 



60 

 6* 

 50 



H/L 



JL. ' " 



PL/TL 



12 3' 



Length (mm) 



Fig. 12 Yoldiella obesa incala. Variation in the ratios of height 

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

 against length of a sample from Incal Sta. CP01 from the Bay of 

 Biscay. 



