A man finds what he thinks are archeological treasures in his backyard and forwards them to the Smithsonian for analysis. To his discovery of a "hominid skull" the Smithsonian responds:
1. The material is molded plastic. Ancient hominid remains are typically fossilized bone.2. The cranial capacity of the specimen is approximately 9 cubic centimeters, well below the threshold of even the earliest identified proto-hominids.
3. The dentition pattern evident on the "skull" is more consistent with the common domesticated dog than it is with the "ravenous man-eating Pliocene clams" you speculate roamed the wetlands during that time. This latter finding is certainly one of the most intriguing hypotheses you have submitted in your history with this institution, but the evidence seems to weigh rather heavily against it. Without going into too much detail, let us say that:
A. The specimen looks like the head of a Barbie doll that a dog has chewed on.
B. Clams don't have teeth.
Alas, it's just an urban legend, albeit a minor comic masterpiece. The full letter and some background details are here.