ECOLOGY— DEPTH DISTRIBUTION 181 



of pedunculate lateral suckers are inserted upon the stalk of the large median sucker, and at the base 

 of the stalk, inserted directly upon the wall of the buccal cavity and almost encircling it, is a single 

 transverse row, consisting on either side of the middle line of five small, sessile suckers. 



Species of Pneumodermopsis appear in general to be of rather restricted distribution. P. paucidens 

 was first described from the tropical Atlantic (Boas, 1886) and Massy (19 17) has recorded it as reaching 

 the south-west coast of Ireland. Pruvot-Fol gives some further subtropical Atlantic localities, and 

 mentions (1942) its occurrence in the Indo-Pacific. 



The considerable amount of material obtained from the Benguela survey yielded several rather 

 well-expanded specimens by which identification was greatly assisted, and it has been thought worth- 

 while to provide a figure (Fig. 6) for this species, since detailed illustrations appear to be altogether 

 lacking. Like all the Gymnosomata, P. paucidens suffers great distortion upon preservation. The foot 

 and wings are frequently lost from sight, disappearing into a roll of skin surrounding the middle of 

 the body like a tyre. The buccal armature is but rarely well-extended. At times, however, even a mis- 

 shapen specimen yields outlines rather characteristic of its species when preserved, and I have pro- 

 vided here a series of outline figures representing the size range and the more typical modes of 

 distortion found in a series of specimens from the larger sample at WS 996 (Fig. 7). 



Class LAMELLIBRANCHIATA 

 Order EULAMELLIBRANCHIATA 

 Two species of lamellibranch larvae occurred at the three sets of inshore stations. The first, lamelli- 

 branch larva 'A', was from WS 981, off Walfisch Bay, where 540 specimens were taken in 0-50 m. 

 The second species, lamellibranch larva 'B', was present off Sylvia Hill, at WS 988 (two larvae at 

 0-50 m., one at 50-100 m.) and at WS 989 (six larvae in 0-50 m.), and, much more numerously, at 

 the mouth of the Orange River where a sample of about 600 was secured at WS 1002 (0-50 m.). 



The Station WS 1002, off Walfisch Bay, is in the area of an 'azoic zone' where the bottom 

 deposit consists of a mud populated with sulphate-reducing bacteria (Currie, 1953 ; Marchand, 1928). 

 It would appear that settlement of the larval lamellibranch taken in large numbers in the plankton 

 hauls is scarcely possible on this ground, and Currie observes that during March oxygen depletion 

 of the water was most marked near Walfisch Bay, water with less than i-o c.c. oxygen per litre 

 extending in some places to the sea surface. 



Lamellibranch larva 'A' (Fig. 8) 



The largest individuals of this species consisted of a ' veliconcha ' made up of the two ' prodisso- 

 conch ' shell stages, and measuring 290-300 fi in length. The smallest members of the population were 

 late stages of the ' D-shaped larva', still possessing the straight-line hinge along about half the visible 

 dorsal margin, and measuring 150-180 /x in length. In neither of the species of lamellibranch larvae 

 were details of the hinge-teeth easily available for examination ; shells preserved in formalin were 

 softened so that their shape became distorted on applying the methods for opening recommended by 

 Rees (1950). Outline figures of the intact shells are, however, provided, in the hope that a further 

 knowledge of lamellibranch larvae of this region or correlation with the occurrence of adults on shore 

 may enable a more exact identification of this material. 



Details of hinge-teeth were available by examination of the transparent intact shell of the late 

 D-shaped or straight-hinged stage. Here, although there is already some differentiation of hinge- 

 teeth, the hinge structure is unfortunately not advanced enough to justify comparison with the stages 

 figured for the main superfamilies by Rees (1950). The provinculum at this stage appeared to bear 

 no teeth at all, but a simple lateral hinge system was present. In the left valve, lateral teeth are 



3-2 



