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Updated: May 5, 2025
I can note only a few points which seem to me to be open to discussion. The Monorhina, having been developed out of the Leptocardia, gave rise, according to Professor Haeckel, to a shark-like form, which was the common stock of all the Amphirhina. From this "Protamphirhine" were developed, in divergent lines, the true Sharks, Rays, and Chimaerae; the Ganoids, and the Dipneusta.
The Pachycardia are then divided into Monorhina, which contains the Cyclostome fishes, distinguished by their single nasal aperture; and Amphirhina, comprising the other Vertebrata, which have two nasal apertures.
So far as our present information goes the Ganoids are as old as the Sharks; and it is very interesting to observe that the remains of the oldest Ganoids, Cephalaspis and Pteraspis, have as yet displayed no trace of jaws. It is just possible that they may connect the Monorhina, with the Sturgeons among the Amphirhina.
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