Dictionary of Ichthyology  

arm = fish do not have arms but placoderms have rigid "arms" in place of the usual pectoral fins. Each arm has a joint where it leaves the body and another a little more than half-way along. The arms are served by interior muscles and the fish may have "walked" on them.

Model of a placoderm, Bothriolepis, Canadian Museum of Nature. Photograph by Brian W. Coad.

Canadian Museum of Nature
www.briancoad.com