As a result of deep-water oil prospecting in the Campos Basin, Brazil, a rich diversity of protobranchiate pelecypods has been revealed. In this study, we identified four new species of Ledella and conducted a morphometric study to corroborate their delimitation. The resulting conchological-based statistic models are in 95% agreement with the traditional criteria for the right valves and 88.33% for the left valves. Certain variables were particularly relevant in these analyses; these variables expressed the importance of the shape of the rostrum and hinge plate for the right valves and the general shell shape and hinge plate for the left valves. The hinge plate variables are not typically used as diagnostic characters of protobranchiate pelecypods and, when applied, they are not given much importance. However, our statistical analyses stressed the importance of these characters. Therefore, we strongly suggest that future descriptions of protobranchiate pelecypods place greater emphasis on these characters.