The most abundant bivalve of the Essex biofacies (Mazon Creek fauna, Pennsylvanian of Illinois), misidentified by past authors as the marine pholadomyoid Edmondia de Koninck, 1841, is herein named Mazonomya mazonenis n. gen., n. sp., and assigned to the family Solemyidae, based on: (1) anterior elongation of the shell as deduced from brevidorsal placement of the hinge-axis, preserved traces of the external ligament, and supporting structures; (2) preserved traces of a longidorsal extension of the ligamental outer layer and periostracum; and (3) sedimentary backfill marks left by the large foot near the longiterminus of the shell. The second most abundant Essex solemyid (Solemya radiata of past authors), showing traces of the periostracil frill and external ligament, is emended as Acharax radiata (Meek & Worthen, 1860) n. comb. Other Essex solemyids previously unreported include two probably solemyids left in open nomenclature, and Acharax (Nacrosolemya) trapezoids (Meek, 1874), for which Meek's original, non-Esses specimen is designated as lectotype. Systematic revisions herein challenge open-marine and open-estuary depositional models of the Essex biogacies. Unlike coeval euhaline oxic communities in which solemyids are the Essex bivalve community is dominated by solemyids, a recurrent phenomenon in carbonaceous roof-strata immediately overlying Pennsylvanian coal seams. Extant solemyids are common in shallow wuryhaline waters, forming dense chemoautotrophic populations in organic-rich dysoxic/ anoxic muds. Within the Essex, the prevalence of solemyids along with an admixture of thin-shelled euryhaline bivalves and growth-inhibited stnohaline 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. Arguably, a persistent suite of traits (amphidetic ligament, edentulous hing, periostracal frill, mantle fusion, reduced gut, and enlarged gills hosting bacterial chemosymbionts) has characterized the Solemyidae since the Early Ordovician. Whereas the dianostic internal ligament of Solemya Lamarck, 1818, is apparently a post-Paleozic trait, the prevalence of external ligaments among Paleozoic solemyids requires that species previously placed in Solemya be transferred to Acharax Dall, 1908, or other genera. Emended examples herein are: S. [Janeia] primeva Phillips, 1836, sensu Hind (1900) (Carboniferous, United Kingdom) is emended as Acharax primaeva n. comb., a probably senior synonym of S. parallela Beede & Rogers, 1899 (Pensylvanian, Kansas) (non S. parallela Ryckholt, 1853 [1854]); Carydium elongatum Clarke, 1907 (Lower Devonian, new Brunswick) is emended as Dystactella elongata n. comb, Additionally, several European Carboniferous species of "Solemya" (e.g., S. puzosiana de Koninck, 1842, S. saginata Ryckholt, 1853 [1854], S. costellata M'Coy, 1844, and S. excisa de Koninck, 1885) should be reassigned to Acharax.