The microstructure of denticles in paired extant and fossil specimens of Callinectes Stimpson, 1860, and Scylla de Haan, 1833, is examined and compared to the cuticle from the remainder of the claw. Denticles of Scylla serrata and Callinectes sapidus are differentiated from the surrounding cuticle by differences in the number of pore canals, tegumental canals, density, microhardness, and phosphorous content. Differentiated denticles can be observed in fossil Scylla and Callinectes on the basis of structure and phosphorus content. Increased hardness of denticle-type cuticle functions to resist abrasion encountered during durophagy and to resist the high forces generated by the denticles as a result of their geometry. Infolds along the lateral margins of the denticles are hypothesized to allow denticles to develop beneath the old cuticle before the molt, and then to emerge and expand after the molt. Future chemical and mineralogical work involving crustaceans should take into account the presence of denticle-type cuticle and not treat the cuticle of chelae as homogeneous tissue. Because denticle-type cuticle can be recognized in the fossil record, the emergence, variation, and phylogenetic trends of this adaptation can be tested. The increased calcification of claw tips and denticles may make them even more durable than the chelae.