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Thus the Devonian Ganoids, though almost all members of the same sub-order as Polypterus, and presenting numerous important resemblances to the existing genus, which possesses biconcave vertebræ, are, for the most part, wholly devoid of ossified vertebral centra.
The Mesozoic Lepidosteidæ, again, have, at most, biconcave vertebræ, while the existing Lepidosteus has Salamandroid, opisthocoelous, vertebræ. So, none of the Palæozoic Sharks have shown themselves to be possessed of ossified vertebræ, while the majority of modern Sharks possess such vertebræ.
The Mesozoic Lepidosteidae, again, have, at most, biconcave vertebrae, while the existing Lepidosteus has Salamandroid, opisthocoelous, vertebrae. So, none of the Palaeozoic Sharks have shown themselves to be possessed of ossified vertebrae, while the majority of modern Sharks possess such vertebrae.
The Ichthyornis type was a tern-like flying bird, with socketed teeth and biconcave vertebrae like the reptile, but otherwise fully evolved into a bird. Its line is believed to survive in the gannets, cormorants, pelicans, and frigate-birds of to-day. The less numerous Hesperornis group were large and powerful divers.
If old, a cinnabar-red streak is seen on drawing a needle across the stain. The human blood-corpuscle is a non-nucleated, biconcave disc, having a diameter of about 1/3500 of an inch. All mammalian red corpuscles have the same shape, except those of the camel, which are oval. The corpuscles of birds, fishes, reptiles, and amphibians, are oval and nucleated.
The primitive biconcave vertebrae of the fish and of the early land vertebrates were retained, and the limbs degenerated into short paddles. The skin of the ichthyosaur was smooth like that of a whale, and its food was largely fish and cephalopods, as the fossil contents of its stomach prove.
A more reptilian character in the Ichthyornis group is the fact that, unlike any modern bird, but like their reptile ancestors, they had biconcave vertebrae. The brain was relatively poor. We are still dealing with a type intermediate in some respects between the reptile and the modern bird. The gannets, cormorants, and pelicans are believed to descend from some branch of this group.
Again, the more ancient Crocodilia and Lacertilia have vertebrae with the articular facets of their centra flattened or biconcave, while the modern members of the same group have them procoelous.
The Mesozoic Lepidosteidae, again, have, at most, biconcave vertebrae, while the existing 'Lepidosteus' has Salamandroid, opisthocoelous, vertebrae. So, none of the Paleozoic Sharks have shown themselves to be possessed of ossified vertebrae, while the majority of modern Sharks possess such vertebrae.
Thus these vertebræ join one another by their edges, like two shallow wine-glasses placed rim to rim. Only gradually is the notochord crowded out so that the vertebræ join by their whole adjacent surfaces. Even in highest forms, for the sake of mobility, they are united by washer-like disks of cartilage. Biconcave vertebræ persisted through the oldest amphibia, reptiles, and birds.
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