United States or Cameroon ? Vote for the TOP Country of the Week !
Lepidotus Mantelli, Agassiz. Wealden. a. Palate and teeth. b. Side view of teeth. c. The fishes of the Wealden are chiefly referable to the Ganoid and Placoid orders. These ganoids were allied to the Lepidosteus, or Gar-pike, of the American rivers. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part coated with enamel.
Hence the original formation of the skeleton of the mouth can be traced to these primitive fishes, from which we have inherited it. The teeth are developed from the skin that clothes the jaws. As a fact, this is found to be the case on microscopic examination of the development and finer structure of the teeth. It is the same with the cutaneous teeth or placoid scales of the Selachii.
Similar bone-beds occur in the Carboniferous Limestone of Armagh, in Ireland, where they are made up chiefly of the teeth of fishes of the Placoid order, nearly all of them rolled as if drifted from a distance. There are upward of seventy other species of fossil fish known in the Mountain Limestone of the British Islands.
Finally, with respect to the 'Vertebrata', the same law holds good: certain types, such as those of the ganoid and placoid fishes, having persisted from the palaeozoic epoch to the present time without a greater amount of deviation from the normal standard than that which is seen within the limits of the group as it now exists.
Finally, with respect to the 'Vertebrata', the same law holds good: certain types, such as those of the ganoid and placoid fishes, having persisted from the palaeozoic epoch to the present time without a greater amount of deviation from the normal standard than that which is seen within the limits of the group as it now exists.
Onchus tenuistriatus, Agassiz. Bone-bed. Upper Silurian. Shagreen-scales of a placoid fish, Thelodus parvidens, Agassiz. Plectrodus mirabilis, Agassiz.
Word Of The Day