Bonnethead Sharks, Sphyrna tiburo, have their mouths on the bottom of their heads, facing the sand. Their large, shovel shaped head is slightly different for males and females. Rows of teeth are designed for crushing bottom-dwelling prey~ this is why the dog (and humans) are in no danger from this shark. The shark swims along the bottom, swinging its head back and forth and sensing electrical vibrations in the sand that might lead to a squid, crab, shrimp, or small fish. The location of the mouth (and comparative size) means that the dog is in no danger, and swimmers are unlikely to be targets.