The structure of the carpus varies widely between different groups of tetrapods, even among those that retain the full set of five digits. In primitive fossil amphibians, such as "Eryops", the carpus consists of three rows of bones; a proximal row of three carpals, a second row of four bones, and a distal row of five bones. The proximal carpals are referred to as the radiale, intermediale, and ulnare, after their proximal articulations, and are homologous with the scaphoid, lunate, and triquetral bones respectively. The remaining bones are simply numbered, as the first to fourth centralia (singular: centrale), and the first to fifth distal carpals. Primitively, each of the distal bones appears to have articulated with a single metacarpal.