DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



27 



Fig. 29 Yoldiella artipica. Lateral view from the right side of the 

 interal morphology of a specimen from Sta. 200 Angola Basin. 

 (Scale = 1.0 mm). For identification of parts see Fig. 5. 



Type specimen. Holotype: BM(NH) 1992023, Paratypes: in 

 collection held by J. A. Allen. 



Fig. 30 Yoldiella similis. Lateral view of a shell from right side and 

 a hinge-plate of a left valve compared with the hinge-plate of a 

 specimen of Y. jeffreysi (a) Specimens from Sta. 197, Angola 

 Basin and Sta. 316, West European Basin, respectively. (Scale = 

 1.0 mm). 



Material 

































Shell measurements (mm 



) and ratios are as i 



bllows:- 





Depth No 



Lat 



Long 



Gear Date 





Cruise Sta 

















(m) 









Length 



Height 



Width 



H/L 



W/L 



PL/TL 



ANGOLA BASIN 











1.0 



0.71 



0.78 



0.71 



0.38 



0.42 



Atlantis II 196 



42 



197 



4612- 1 

 4630 

 4596 35 



10°19.0'S 



9°04.0'E 



ES 21.5.68 



1.93 



1.72 



1.39 

 1.43 



0.80 

 0.80 



0.72 

 0.83 



0.41 

 0.46 



0.50 

 0.54 



10°29.0'S 



9°04.0'E 



ES 21.5.68 



2.90 



2.06 



1.39 



0.71 



0.48 



0.55 



198 



4559- 12 



10°24.0'S 



9°09.0'E 



ES 21.5.68 

















4566 



- 10°29.0'S 



9°04.0'E 





PL = postumbonal length 









Found at abyssal depths in the Angola Basin. Depth range: 

 4559-4630 m. 



Shell description (Fig. 30). Shell small, ovate, very 

 slightly inflated, inequilateral with posterior margin suprame- 

 dially subrostrate, shell with very fine concentric lines, form- 

 ing ridging on ventral half of shell; periostracum pale yellow; 

 umbos slightly raised, internally directed; dorsal margin 

 slightly convex, antero-dorsal margin slopes from umbo to 

 form even curve with anterior margin, ventral margin 

 smoothly curved, posterior margin sharply curved where it 

 meets postero-dorsal margin; hinge strong, moderately 

 broad, extends to anterior and posterior limits of the anterior 

 and posterior adductors respectively; hinge teeth well- 

 developed, equal numbers (maximum 9) on anterior and 

 posterior hinge plates, ligament amphidetic, moderately 

 large, goblet-shaped with very small external extensions. 



Prodissoconch length: 173 (im. Maximum recorded shell 

 length: 2.92 mm. 



The shell outline of this species is similar to that of 

 specimens of Y. jeffreysi from the Cape Verde Basin. The two 

 species can be distinguished by the larger ligament, broader 

 hinge and more rostrate posterior margin of Y. similis. 



Internal morphology (Fig. 31). The internal morphology 

 of Y. similis is very similar to that of Y. jeffreysi. The exhalent 

 siphon is combined with the less well-developed inhalent 

 siphon. The adductor muscles are unequal in size, the ante- 

 rior being approximately three times larger than the poste- 

 rior. In lateral view the posterior adductor muscle is oval in 

 outline while the anterior is 'bean-shaped'. The foot has a 

 long, extended, narrow sole with a large byssal gland in the 

 heel. The nervous system is extremely well-developed with 

 large visceral and cerebral ganglia with short stout commis- 

 sures joining them. Gills appear to have relatively few plates 

 (up to 10) and the number of palp ridges is also few (up to 12) 

 but the latter are relatively broad. This species has a large 

 stomach and the hind gut makes one complete double coil to 

 the right side of the body. 



Yoldiella sinuosa (new species) 



Type locality. R.V. Knorr, Cruise 25, Sta. 299, Surinam 

 Basin, 29.2.1972, 7°55.1'N, 55°42.0'W, Epibenthic Trawl, 

 1942-2076 m. 



