The Elasmobranchs are characterised by the fact that the gill slits open individually to the exterior, there being no gill cover, such as is found in the other groups. Their scales are simple, tooth-like projections, and in fact there is no essential difference between them and the teeth. The skull is more primitive than in the other groups, but a discussion of its details would necessarily be very involved. The living members of the group show a fairly high stage of development of the vertebræ—considerably higher, in fact, than that found in the lung fishes—but some extinct members showed a very primitive condition with regard to this point. In the fossil skeleton shown in Fig. 71, for instance, it is apparent that the notochord was present as a simple continuous rod. The skeleton in question is from the Permian and belongs to what is regarded as the most primitive type of fish known. Two specimens of Elasmobranchs are shown in Figs. 72 and 73, and the teeth of a shark in Fig. 74.
Fig. 71.—Fossil skeleton of Pleuracanthus Decheri, a primitive Elasmobranch.
Photo: Thiele.
Fig. 72.—Spotted dog-fish.
Photo: Thiele.