130 



J.A. ALLEN AND H.L. SANDERS 



Fig. 45. Propeleda paucistriata. a, internal morphology as seen from the right side: b, part of the left side of the same specimen to show details of the 

 course of the gut; c, the internal morphology as seen from the left side of a much larger specimen. All specimens taken from Atlantis II station 203 from 

 the Angola Basin. For identification of the parts see text-figure 7, p. 106. Scales = 1mm. 



and extend from the posterior visceral mass to the anterior limit of 

 the posterior adductor. There are at least 22 gill plates in larger 

 specimens. A slender extension of the axis extends from each gill 

 from ventral to the posterior adductor to the inner junction between 

 inhalent and exhalent siphons. 



From the large combined stomach and style sac the course of the 

 hind gut takes it first dorsal and posterior to the stomach and then to 

 the left side of the body where it makes a small loop. From there it 

 passes ventral to the umbo to the right side of the body where it 

 makes a much larger loop at the perimeter of the viscera and passing 

 close to the inner face of the anterior adductor. From there it passes 

 mid-dorsally to the anus. The mouth is set some distance posterior to 

 the anterior adductor muscle. The oesophagus is broad and elongate 

 and the combined stomach and style sac is positioned vertically in 

 the posterior part of the visceral mass. The pedal ganglion is large 

 and lies immediately anterior to the junction of mid gut and hind gut. 



The shell surface in some larger specimens is covered with 

 epifaunal solitary hydroids. This would indicate that P. paucistriata 

 lives close to the surface of the sediment. This is also suggested by 

 the fact that the posterior tips of the shells of larger specimens are 

 broken. We believe that this is the result of predation on the siphons. 



The extreme post-adductor elongation of the shell is advanta- 

 geous in that it provides distance between predator and the more 

 vulnerable viscera with damage being restricted to more easily 

 generated tissue. 



The shell of this species differs from others described by the small 

 number of pronounced concentric shell ridges and we name it with 

 reference to this characteristic feature. This is the first species of 

 Propeleda to be recorded off the south-west coast of Africa. At 

 approximately the same latitude off the east coast of Natal a species 

 named Leda lanceta by Boshoff ( 1 968) occurs at upper slope depths. 

 Nijssen-Meyer (1972) believes that this latter species is a Propeleda, 



and we concur with her. P. lanceta is more robust, more arcuate and 

 with far more numerous concentric ridges than is the case in P. 

 paucistriata. 



DISCUSSION 



The major point of interest in this particular account of deep-sea 

 protobranch bivalves is the evidence it provides to further our 

 understanding of the evolution of the nuculanoid protobranchs. In 

 our earlier studies on the Tindariidae we speculated as to how the 

 nuculanoids could have evolved from the nuculoids (Sanders and 

 Allen, 1977). In functional terms, this involved a change in the 

 inflow of water into the mantle cavity from an antero-ventral posi- 

 tion to a posterior position and the begining of specialization of the 

 posterior mantle edge, a view also expressed by Yonge ( 1 939). In the 

 tindariids this latter involves the development of sensory papillae 

 from the sensory fold of the mantle at the points of ingress and exit 

 of the circulatory water. Although infaunal and deposit feeding, the 

 tindariids, like the nuculids, live close to the surface and, like many 

 other bivalves that occupy this position, they are ovate and robust. 

 Many of these subsurface dwelling bivalves, including the tindariids, 

 have stout external ligaments. 



The development of siphons was the next step in the evolutionary 

 process and the neilonellids are illustrative of this. The shell form and 

 ligament as seen in the tindariids is largely retained in the neilonellids, 

 but short siphons, as yet only fused dorsally, are now present and these 

 are contained in a shallow siphonal embayment. Although the shell 

 remains stout and ovate there is some posterior elongation and an area 

 ventral to the inhalent siphon from where the palp proboscides are 

 extended is more defined. Like the tindariids the neilonellids are 

 deposit feeders living close to the surface of the sediment. 



