Trilobite

tri'lo-bit, one of a group of animals extinct since the coal age. Over 500 species have been described. They are found chiefly in limestone. The fossils are generally of a flattened oval figure, presenting three ridges or lobes running lengthwise, whence the name. They had compound eyes. Scientists are uncertain whether they are the fossils of animals related to crayfish or spiders, but it is believed that their nearest relatives now living are the horseshoe crabs.