1. The relations to the afferent pseudobranchial artery of the lower ray-bearing part of the hyoid arch skeleton in Rays show that the latter is not the ceratohyal, but a pseudohyoid.

2. Investigation of the development of Torpedo shows that the pseudohyoid arises distinct from the hyoid bar, and that the ceratohyal is much reduced.

3. The ceratohyal in Rhynchobatus is not so much reduced as in Torpedo, Pristis, or Raja, and it still bears a small number of hyal rays.

4. The pseudohyoid is formed as the result of fusion of hyal rays, and pseudohyoid bars are present more or less well developed in a large number of Sharks.

5. The enlargement of the pseudohyoid and reduction of the ceratohyal in Rays is associated with the increased distance between the mouth and the first gill-slit, and the necessity for providing a support for the hyal rays and the anterior wall of the first gill-slit. Similar factors have been operative in the Dipnoi and have led to analogous results.

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