The mandibles of the Cockroach are powerful, single-jointed[50] jaws, each of which is articulated by a convex “condyle” to the lower end of the epicranial plate, and again by a concave “ginglymus” to the clypeus. The opposable inner edges are armed with strong tooth-like processes of dense chitin, which interlock when the mandibles close; those towards the tip of the mandible are sharp, while others are blunt, as if for crushing. Each mandible can be moved through an angle of about 30°. A flexible chitinous flap extends from its inner border to the labrum. The powerful flexor of the mandible arises within the epicranial vault; its fibres converge to a chitinous tendon, which passes outside the central plate of the tentorium, and at a lower level through a fold on the lower border of the clypeus, being finally inserted near the ginglymus. A short flexor arises from the crus of the tentorium. The extensor muscle arises from the side of the head, passes through the fold formed by the lower end of the gena, and is inserted close to the outer side of the condyle of the mandible. [50] Professor J. Wood-Mason points out that in _Machilis_ (one of the Thysanura) the mandible shows signs of segmentation, while the apical portion is deeply divided into an inner and an outer half. Ripe embryos of _Panesthia (Blatta) javanica_ are said to exhibit folds which indicate the consolidation of the mandible out of separate joints, while the cutting and crushing portions of the edge are divided by a “sutural mark,” which may correspond to the line of junction of the divisions of a biramous appendage (Trans. Ent. Soc., 1879, pt. 2, p. 145).