A structure resembling the lacinia mobilis in Peracarida (Malacostraca) is described for the coxal gnathal edge of the mandibles in the halocyprid ostracode Spelaeoecia capax. The lacinia mobilis is present at the distal portion of the gnathal edge close to and articulated with the incisor process. Proximal and closely associated with the lacinia, two processes are present. In the center of the gnathal edge, a flat process partly covers two comb-like spines. A molar process (masticatory pad) is present at the proximal end of the gnathal edge. The lacinia mobilis described herein resembles the peracarid lacinia in possessing a number of cusps similar to the cusps of the incisor process, its orientation parallel to the incisor process and the presence of an articular condyle at the basis of the lacinia. The described similarities, however, concern only the left mandible of certain Peracarida where the mandibles are asymmetrical. In Spelaeoecia capax, the mandibles are symmetrical. The lacinia mobilis in certain ostracodes may have evolved independently from that in Peracarida. The term lacinia mobilis is used as a descriptive term only, not implying homology.