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.

Eight species ofYoldiella(Yoldiella biguttataAllen, Sanders & Hannah, 1995;Yoldiella similisAllen, Sanders & Hannah, 1995;Yoldiella extensaAllen, Sanders & Hannah, 1995;Yoldiellaaff.jeffreysi(Hidalgo, 1877);Yoldiellasp.1;Yoldiellasp. 2;Yoldiellasp. 3 andYoldiellasp. 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 ofYoldiellaspecies is subtle, and also the findings of this study, we can state that the hinge plate morphometry has good potential to improve species discrimination.

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Subjectivity is a common problem in most taxonomic studies that deal with gradual changes in shape. An example is the systematics of the genus Yoldiella, which has been a matter of debate for over 40 years. However, hinge plate features have never been examined as a significant diagnostic feature in descriptions of Yoldiella species. We tested how hinge plate characters perform in discriminating among three species of this genus from the Campos Basin, Brazil. We tested the efficacy of traditional shell shape features alone, and contrasted this with a general analysis that included the hinge plate features; and also with an analysis based solely on the hinge plate. Some information on the hinge plate is used here for the first time, such as the distance between the highest teeth to the umbo and the distance between the highest teeth to the margin. The hinge plate analysis and a general analysis, including both shell shape and hinge plate variables, were more effective (87.5% of cases assigned correctly) than the analysis that included only shell shape variables (73.3% cases assigned correctly). The variables related to the highest teeth and the width of the hinge teeth stood out as major contributors in discriminating among these three species.

As a secondary result of oil prospecting in Brazil, samples from the Campos Basin continental slope became available. In these samples, specimens of the genera Microgloma Sanders & Allen, 1973 and Pristigloma Dall, 1900 were found. This contribution provides the southernmost record of the genus Microgloma, the first record of Microgloma mirmidina (Dautzenberg & Fischer 1897) from the western Atlantic, the descriptions of Microgloma macaron sp. n. and Microgloma nhanduti sp. n.as new species, and the shallowest record of Pristigloma alba Sanders & Allen 1973.