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.

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