A specimen of the ammonoid Cleviceras (Hildoceratidae), collected at Port Mulgrave, near Whitby, North Yorkshire, shows an in-situ aptychus in the body chamber. The precise stratigraphic origin of the specimen is unknown, but the lithology and generic identity are compatible with derivation from the Mulgrave Shale Member of the Whitby Mudstone Formation. The specimen is the first in-situ jaw apparatus to be reported for this genus from Britain. A second loose specimen, a Harpoceras sp. with in-situ aptychus was found and photographed at Deepgrove Wyke, but was too friable to be collected. Both specimens reinforce previous observations that the aptychus form type Cornaptychus is characteristic of the ammonite family Hildoceratidae.