Recent years have seen a growing interest in the direct and indirect roles of organisms, and more explicitly of organic matter, in the crystallization of minerals in sediments. Two bivalve genera form extensive biofilms that are apparently responsible for the growth of a variety of isolated crystals together with a marine cement. Granicorium indutum and Samarangia quadrangularis are infaunal species that cover their shells with a cemented coating of sand, sculpted to mimic the surface ornament typical of many bivalves. Granicorium has an exceptionally mobile mantle margin that produces large volumes of mucus, harboring a diverse microbial community. The sand grains surrounding both species are initially bound by a biofilm that provides structural integrity but also acts, like others, as a template for the crystallization of a variety of carbonate polymorphs. These include varied prismatic crystals, rice-grain and wheatsheaf forms and, more importantly, large volumes of acicular crystals indistinguishable from typical marine cements. The distribution of these crystals is related to positions accessible to the mantle of the animal and in Samarangia appears to result from the active emplacement of grains and mucus several times a year. Summing the evidence, it seems that these species are responsible not only for the biomineralization involved in the formation of their shells but also for crystallization mediated through biofilms and for the generation of cements that are morphologically indistinguishable from typical marine cements. The boundaries between these three strategies may be closer than we think.