Eight species of Yoldiella (Yoldiella biguttata Allen, Sanders & Hannah, 1995; Yoldiella similis Allen, Sanders & Hannah, 1995; Yoldiella extensa Allen, Sanders & Hannah, 1995; Yoldiella aff. jeffreysi (Hidalgo, 1877); Yoldiella sp.1; Yoldiella sp. 2; Yoldiella sp. 3 and Yoldiella sp. 4) from the continental slope off Rio de Janeiro were used to test if quantitative morphometric measurements of shell shape and hinge plate could effectively discriminate among them. Thirty specimens of each species were sampled, and a total of 25 variables were established and utilized as input data to perform a discriminant analysis. The percentage of correctly classified cases was never less than 80%. The hinge plate variables were always relevant, and the most important one was the width of the posterior hinge plate. On the other hand, shell shape variables, when present, were secondary. Considering that the variation in the shell of Yoldiella species is subtle, and also the findings of this study, we can state that the hinge plate morphometry has good potential to improve species discrimination.

Despite the increasing number of reports on the deep-sea molluscs from the south-western Atlantic, we know very little about the protobranchs. The lack of information on the protobranch Pelecypoda off southern Brazil is reflected in the genus Yoldiella. This contribution is part of an effort to increase the knowledge about this group off the Brazilian coast. Eight species of Yoldiella are recognized here. For Yoldiella biguttata, previously reported from Brazil, the known distribution is extended southwards to the Campos Basin. For Yoldiella similis this is the first record in the western Atlantic Ocean. For Yoldiella extensa and Yoldiella aff. jeffreysi this is the first record for Brazil. Four previously unknown species are described, Yoldiella lapernoi sp. nov., Yoldiella paranapuaensis sp. nov., Yoldiella arariboia sp. nov. and Yoldiella curupira sp. nov. Considering only conchological features for the Atlantic species we could propose some clusters of species of Yoldiella.