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Of the old type of Selachians we have again one lingering representative in our own times to give us the clue to its ancestors, as the Gar-Pike explains the old Ganoids, and the Chambered Nautilus helps us to understand the Chambered Shells of past times.
In the letter above quoted, he even named the species he expected to find most prevalent in those greater depths: as, for instance, representatives of the older forms of Ganoids and Selachians; Cephalopods, resembling the more ancient chambered shells; Gasteropods, recalling the tertiary and cretaceous types; and Acephala, resembling those of the jurassic and cretaceous formations.
The Selachians resemble higher Vertebrate types not only in the small number of their eggs, and in the closer connection of the young with the mother, but also in their embryological development, which has many features in common with that of birds and turtles.
These are the Sharks and Skates, or, as the Greeks used to call them, the Selachians, making a very appropriate distinction between them and common fishes, on account of the difference in the structure of the skeleton.
Fishes abound, but their class still consists, as before, of the Ganoids, those fishes of the earlier periods built on the Gar-Pike and Sturgeon pattern, and the Selachians, represented now by Sharks and Skates. In the Carboniferous period we begin to find perfectly preserved specimens of the Ganoids, and the adjoining wood-cut represents such a one.
Whether this living picture of old creations in modern garb was meant to be educational for man or not, it is at least well that we should take advantage of it in learning all it has to teach us of the relations between the organic world of past and present times. There were a great variety of the Selachians in the Carboniferous period.
In Selachians the quality of the bones is granular, instead of fibrous, as in fishes; the arches above and below the backbone are formed by flat plates, instead of the spines so characteristic of all the fish proper; and the skull consists of a solid box, instead of being built of overlapping pieces like the true fish-skull.
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