Middle Miocene coral-oyster patch reefs crop out at Murchas, south of the city of Granada in southern Spain. They are irregularly shaped masses of coral-oyster boundstone, up to 18m wide and 3-4 m high, that developed on the outer part of a homoclinal ramp, seaward of some sand shoals, in a mixed carbonate-terrigenous enviroment. In these patch reefs, oysters and hermatypic corals are the main frame-builders, their association being entirely fortuitous. Heliastrea is the predominant coral. Porites, Tarbellastraea and the phaceloid coral Mussismilia are also important components. These corals show no clear pattern in their distribution and appear embedded in a silty (bioclastic) matrix. Oysters in the reef community belong to the species Hyotissa squarrosa. They grew vertically one upon another, anchored directly to coral skeletons or, more commonly, attached to other oysters. Hyotissa is irregularly distributed but in places accounts for up to 70% of the reef. Encrusting organisms are restricted to sediments between individual coral colonies or between reefs.