Malacology means the study of molluscan animals and also conchology means the study of the molluscan shell. Body mass is one of the best known and most studied characteristics of aquatic animals on scaling of metabolic rates. We studied here how size-specific variation in the rate of oxygen consumption, ammonia excretion, and O: N ratio in Freshwater Bivalve Mollusc Lamellidens marginalis species in an attempt to know how size-specific variation affects their metabolism. The freshwater bivalve molluscs were chosen for experimental work from Bhima River at Siddhatek in August and September for the period of monsoon season with body size i.e. small (75-79 mm in shell-length) and large (90-93 mm in shell-length). In current work reported that the rate of oxygen consumption and O: N ratio was high in the small body-sized bivalve mollusc but the rate of ammonia excretion was low in small body-sized bivalves compared to large ones. The results are discussed in the flush of metabolic processes in fresh-water bivalve molluscs.