of fishes, which, according to Cuvier, exceeded 5000; but are stated by M. Agassiz to amount to 8000. On the contrary, the greater number of fossil fishes belong to the two orders with enamelled scales. In the following table the geological distribution of these orders is sketched.
| Placoid | Ganoid | Ctenoid | Cycloid | |
| Living | ✽ | * | ✽ | ✽ |
| Tertiary | ✽ | * | * | * |
| Cretaceous | ✽ | * | ||
| Oolitic | ✽ | ✽ | ||
| Saliferous | ✽ | * | ||
| Carboniferous | ✽ | * | ||
| Silurian | ✽ | * |
Among existing fishes it is frequently found that the caudal tail fin divides into two equal branches; sometimes it is single and rounded, but in the case of some placoid and ganoid fishes (e. g. squalus and lepidosteus) the tail fin is double, the dorsal portion being prolonged to a considerable length, and the ventral portion much shorter. These three forms are seen in figs. 27, 28, 29., which represent the trout, the
wrasse, and the shark. Now it is a remarkable circum-
