DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



Material: 



Cruise Sta 



Depth No 



(m) 



Lat 



Long 



Date 



Gear 



ANGOLA BASIN 

 Atlantis II 203 



42 



527- 31 

 542 



08"48.00'S 



12"52.00E 



23.5.68 



ES 



Distribution. Restricted to the Angola Basin at upper slope 

 depths, 527-542 metres. 



Shell description (Fig.44) 



Elongate, fragile, transluscent shell, moderately slender, two cari- 

 nate ribs from umbo to posterior margin, widely spaced prominant, 

 relatively broad, concentric ribs with overhanging ventral margin, 

 2—4 fine concentric lines between ribs, between carinae vertical ribs 

 and lines equally prominant; umbo moderately raised, far anterior in 

 largest specimens (post-umbonal length 79% of total length) but less 

 so in smaller specimens, beaks inwardly facing; antero-dorsal mar- 

 gin sloping, proximally convex but almost straight in small specimens, 

 joins with anterior and antero-ventral margins in smooth curve; 

 postero-ventral margin very slightly sinuous, posterior margin usu- 

 ally damaged in large specimens, intact margin angled and sinuate, 

 forming a hook dorsally where postero-dorsal margin and dorsal 

 carina meet, postero-dorsal margin concave, proximally raised, 

 elongate escutcheon outlined by dorsal carina; hinge elongate, 

 moderately broad, large, acute chevron-shaped teeth up to 16 in 

 anterior series and up to 28 in posterior series, anterior series extends 

 to anterior limit of anterior adductor muscle, posterior series extends 

 approximately half length of postero-dorsal margin to anterior limit 



129 



of posterior adductor muscle, ventral margin of hinge plate corre- 

 sponds to line of dorsal carina, ventral to umbo teeth approach shell 

 margin, anterior and posterior hinge plates continuous; ligament 

 internal, ventral to umbo and close to shell margin, slightly inclined 

 posteriorly; rounded internal ridge extends from umbo to posterior 

 margin. The maximum recorded shell length is 14.1mm. The 

 prodissoconch is very large and measuring 560pm in length. 



Juvenile shells are more ovate and, before posterior elongation 

 occurs, could be mistaken for a yoldiellid (Fig.44). 



Internal morphology (Fig.45) 



The anterior adductor muscle is oval in cross section, while the 

 posterior adductor is smaller and more elongate. Both are set in from 

 the shell margin, the posterior is positioned at approximately two- 

 thirds the distance between the umbo and the posterior limit of the 

 shell. There is a small anterior sense organ formed from the sensory 

 fold of the mantle, ventral to the anterior adductor. The siphons are 

 joined with their ventral margins fused to form entire lumina. They 

 are slender and particularly elongate and when contracted are con- 

 tained in the elongate siphonal embayment. The foot and viscera lie 

 in the anterior half of the mantle cavity. The foot is elongate and 

 directed anteriorly. In most preserved specimens the tip of the foot 

 lies between the anterior adductor and the shell margin. The margins 

 of the sole are fringed with numerous relatively small papillae. 

 There are three anterior and two posterior pedal retractor muscles. 

 The palps and gills are markedly narrow and elongate. The are at 

 least 22 palp ridges in the largest specimens and the palp probosci- 

 des are attenuate each with a straight dorsal margin and a papillate 

 ventral margin. In a few preserved specimens the palp proboscides 

 extend from the feeding aperture. The gills are similarly attenuate 



Fig. 44. Propeleda paucistriata. External lateral views of the right sides of four shells of differing size to show change of shape with growth and an 

 internal view of a right valve. All specimens taken from Atlantis II station 203 from the Angola Basin. Scale = 1mm. 



