In the British Pennines, from the middle of the lenisulcata Zone to the lower part of the communis Zone (Upper Carboniferous, lower Westphalian A), nine horizons, of which six are new or newly described, yield elongate specimens of the bivalve Carbonicola with evidence of former steep, near vertical burrowing in sediments of low organic carbon content. The faunas, of the groups of C. bellula (Bolton) sensu lato and of elongate C. aff. communis Davies & Trueman, were evidently invaders of the Pennine prodelta. They grade with established faunas of prolific highly variable Carbonicola found typically in richly carbonaceous shales and characterised by shallow burrowing, greater size, increased prcportionate shell height and length of anterior end, and probably increased obesity. Parallels exist with work on Anthraconaia. Anthraconauta. and the ecology of recent Unionacea. Results also bear on the palaeoecological ranges of Curvirimula and Naiadites.