H. Hippopus and H Porcellanus are almost identical.H.Hippopus is as far as i know the only Hippopus clam available in the hobby.H.Porcellanus is much more rare,and I've never seen it for sale.H.P has a fringed incurrent syphon,where H.H doesn't,also H.H has a smoother shell.thats about all i can tell you for now .if you can get a very close up pic of the clam from every angle then maybe I can tell for sure.