Your species may be Meretrix meretrix, this species have morphological variation among sub species and you can follow - A revision of the Indian species of Meretrix by James Hornell.
Doctor Yambem Tenjing I am sure it is a Mactridae or a young phenotype of Mactra antiquata Spengler, 1802 of India. I send you a photo and a publication.
The specimen shown by Roberto appears to be a Polymesoda, similar to the east coast and Gulf of Mexico species Polymesoda caroliniana in North America found in salt marshes and mangroves.
The specimen Yambem has uploaded is interesting. I agree it is not a Meretrix. I also wonder are the valves equal, because the hinge structure, pallial sinus, and overall shape and size seem consistent with something in the Corbulidae. The key would be inequal valves.