By Harold McGee
The number of different kinds of fish in the world is staggering. Of all the animals that have backbones, fish account for more than half, something approaching 29,000 species. Our species regularly eats hundreds of these. Perhaps two dozen are at least occasionally available in U.S. supermarkets, and another several dozen in upscale and ethnic restaurants, often under a variety of names. The box surveys the family relations of some commonly eaten fish, and the paragraphs that follow provide a few details about the more important families.