Published as part of Hammer, Øyvind, Hryniewicz, Krzysztof, Hurum, Jørn H., Høyberget, Magne, Knutsen, Espen M. & Nakrem, Hans A., 2013, Large onychites (cephalopod hooks) from the Upper Jurassic of the Boreal Realm, pp. 827-835 in Acta Palaeontologica Polonica 58 (4) on page 833, DOI: 10.4202/app.2012.0020, http://zenodo.org/record/13278681

Fig. 9. Functional morphology of a hook or claw in a logarithmic spiral shape, penetrating a surface. A, B. Thrusting action, pole proximal. The angle φ at the contact point and the direction of the force vector (arrow) are constant during penetration. C, D. Pulling action, pole distal.