What is a cartilaginous fish?

Nationalnyckeln (NN) is an encyclopedia dealing with various groups of animals and plants found in the Nordic countries, principally Sweden. NN is a work in progress, and currently 17 volumes have been published. Originally, NN was supposed to become a mammoth work covering all (!) species of organisms found in Sweden, but it's not clear at the present time whether this goal will ever be reached.

This volume, codenamed DZ 1-34, was intended as the first one covering chordates (including vertebrates), and it therefore contains an extensive introduction to this group of creatures. This explains the otherwise curious presence of dinosaurs and other seemingly irrelevant animals in a book supposedly about fish!

Yes, fish. “What *is* a fish” wonders NN at its homepage, and after leafing through this volume, I can only concur with the questioner. Some of the creatures included are positively disgusting, such as the lampreys and the hagfish. Others are spectacular: the blue shark, the Greenland shark and the basking shark are among those. The volume also describes tunicates, which despite their anomalous looks are evolutionarily closer to fish than to invertebrates. For those interested in such things, here follows an exact breakdown of the contents: lancelets (1 species), tunicates (53 species), hagfish (1 species), lampreys (3 species), cartilaginous fishes (29 species).

Yes, it's in color. And no, it's not really bilingual, the only English texts being in the identification keys and very short species summaries. Still, deserves five stars.