What is the X in X-mas?
Any of a mostly extinct group of lobe-finned fishes of the order Crossopterygii, whose only living member is the coelacanth. One group of crossopterygians is thought to have evolved into terrestrial vertebrates beginning in the Devonian Period. See more at coelacanth.
any member of the subclass Crossopterygii, a group of primitive, lobe-finned, bony fishes believed to have given rise to the amphibians and all other land vertebrates. They appeared at the beginning of the Devonian Period (395,000,000 years ago) but are now represented only by the coelacanth (Latimeria chalumnae).