The most abundant bivalve of the Essex biofacies (Mazon Creek fauna, Pennsylvanian of Illinois), misidentified by past authors as the marine pholadomyoid
Edmondis de Koninck, 1841, is herein named
Mazonomya mazonensis n. gen., n. sp., and assigned to the family Solemyidae. The second most abundant Essex solemyid
(Solemya radiata of past authors), showing traces of the periostracal frifi and external ligament, is emended as
Acharax radiata (Meek & Worthen, 1860) n. comb. Systematic revisions herein challenge open-marine and open-estuary depositional models of the Essex biofacies. Within the Essex, the prevalence ofsolemyids along with an admixture of thin-shelled euryhaline bivalves and growth-inhibited stenohaline bivalves is suggestive of a transitional paleoenvironment, such as a drowned coal-swamp or restricted estuary, in which superabundance of organics and nutrient pollution had induced eutrophication.
