DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



79 



inflated, ovate, inequilateral; umbos inflated, orthogyrate, 

 anterior to midline; proximal dorsal margin slightly convex, 

 antero-dorsal margin convex, slightly angulate at limit of 

 hinge plate, then relatively straight section to dorsal limit of 

 anterior margin, antero-ventral margin and ventral margin 

 smoothly curved, posterior margin drawn out into broad 

 rounded medial tip, postero-dorsal margin slightly convex 

 sloping gradually towards tip, slight dip in outline at limit of 

 hinge plate, posterior margin slightly rostrate; no marked 

 rostral ridge but a small lunule and escutcheon present close 

 to umbos; hinge plate moderately broad, long strong teeth, 

 12 in posterior and 10 in anterior series in largest individual, 

 ligament internal, amphidetic, moderately large, 'goblet'- 

 shaped, short anterior and posterior external extensions of 

 fused periostracal. 



Maximum observed shell length: 3.86 mm. 



Internal morphology (Fig. 129). Well-developed com- 

 bined inhalent and exhalent siphons are present, the inhalent 

 siphon is somewhat the shorter than exhalent. The siphonal 

 tentacle is usually on the left side. The anterior sense organ is 

 well-developed. The adductor muscles are relatively small. 

 The anterior adductor is 'cresent'-shaped and approximately 

 twice the size of the oval posterior adductor. 



The gills have up to 14 alternating filaments. The labial 

 palps are small, extending over approximately 1/4 distance of 

 body and have up to 9 inner palp ridges. The palp probos- 

 cides are long and thin. The visceral ganglia are relatively 

 slender, the cerebral ganglia are larger and more oval in 

 shape and the pedal ganglia are large and round. The foot is 

 moderate in size with a large byssal gland. There is a 

 relatively large stomach with the style sac ventral and slightly 

 posterior to it. The hind gut penetrates deep into the foot 

 ventral and anterior to the pedal ganglia before turning 

 dorsally to umbonal region where it passes to the right side of 

 the body and forms a single loop. The hind gut has a 

 typhlosole along its length. 



This species is similar in shell shape and internal morphol- 

 ogy to Portlandia minuta but differs from the latter in that P. 

 fora has a less angulate shell margin, slightly larger internal 



Fig. 129 Portlandia fora. Lateral view from the right side of the 

 internal morphology of a specimen from Sta. S 56, West 

 European Basin. (Scale = 1.0 mm). For identification of parts see 

 Fig. 34. 



ligament, a more obvious external ligament, is less inflated 

 and has a greater number of hinge teeth, the hinge plate is 

 narrower, and the post-umbonal length shorter. Anatomi- 

 cally there are relatively few differences, the gill plates and 

 palp ridges are marginally fewer in specimens of a similar 

 size. 



Portlandia minuta (new species) 



Type location. R.V. Atlantis, II Cruise 42, Sta. 203, 

 Angola Basin, 23.5.1968, 8°48'S, 12°52'E, Epibenthic Trawl, 

 527-542 m. 



Type specimen. Holotype BM (NH) 1992040. Paratypes: in 

 collection held by J. A. Allen. 



Material. 



Cruise 



Sta 



Depth No Lat 

 (m) 



Long 



Gear Date 



CAPE BASIN 



Atlantis II 188 619-622 33 23°00.0'S 12°58.0'E ES 16.5.68 



ANGOLA BASIN 



Atlantis 42 203 527-542352 8°48.0'S 12°52.0'E ES 23.5.68 



Occurs off S.W. Africa at upper slope depths in Angola & 

 Cape Basins. Depth range: 527-622 m. 



Shell description (Figs. 130 & 131). Shell small, inflated, 

 subovate, posteriorly narrow, robust, fine but somewhat 

 irregular concentric lines; slightly iridescent, pale yellow 

 periostracum; umbos inflated, anterior to midline, internally 

 directed; lunule and escutcheon barely visible; slightly ros- 

 trate, slight indication of rostral ridge in some specimens; 

 antero-dorsal margin convex, slopes rapidly and evenly to 

 anterior margin, postero-dorsal margin slightly convex, 

 slopes gradually to posterior margin, slight angle at limit of 

 posterior hinge plate, ventral margin smoothly curved, cen- 

 trally deep, posterior margin drawn out but moderately 

 rounded, in mid horizontal plane; hinge plate strong, fairly 

 narrow on either side of ligament, broadens out distally, 

 distal teeth prominent, 3-4 small proximal teeth, in total 9 in 

 anterior series and 11 in posterior series of largest specimen; 

 ligament amphidetic, rectangular in shape, small external 

 secondary extensions of fused periostracum on either side of 

 umbo. 



Prodissoconch length: 166 p.m. Maximum recorded shell 

 length: 2.28 mm. 



Internal morphology (Fig. 132). There is a combined 

 siphon with a single lumen, thus there is no separation 

 between inhalent and exhalent lumena, however, the gill axes 

 join mid-laterally. Gland cells are present at the junction of 

 axis and siphon. There is a well-developed elongate anterior 

 sense organ. The adductor muscles are unequal in size. The 

 smaller posterior muscle is oval in shape, the anterior is 

 almost twice the size and 'cresent'-shaped. The gills are 

 well-developed with up to 14 relatively large plates, the most 

 posterior of the inner plates are clearly interlocked even in 

 the preserved specimens. The labial palps are relatively small 

 with up to 8 palp ridges, the most posterior of these being 

 much broader than the rest. 



The foot is large but the pedal musculature is not as 

 well-developed as in some Yoldiella species. There is a large 



