We found these critters at Ocean Beach in Connecticut. I asked at Mystic Aquarium for identification: Mole Crab. They look like a shrimp without the long tail or fan. Very cool to watch them dig through the sand to bury themselves.

Dorso-ventrally compressed body; large, triangular plate-like pleotelson (last abdominal segment) and posterior ventral gills (both are most visible in ventral image), and body divided into three zones - cephalothorax, pereon, and pleotelson - reminds me of a marine isopod. Quite a diverse group.