Abstract
1. An attempt has been made to determine the functional significance of the hypocercal tail in the swimming of one of those Ostracoderms which possessed it, Pteraspis rostrata. 2. Since Pteraspis was almost certainly denser than the medium in which it lived, a dynamic upthrust (lift) would have to be generated by forward motion. It is suggested that this lift was obtained by an inclination of the whole body of the animal to act as a lifting plane during forward movement. 3. The hypocercal tail would depress the hinder end of the body, so giving rise to the required inclination. This, it is suggested, was its functional significance in Pteraspis. 4. The above would apply to all those Ostracoderms (Anaspida and Heterostraci) with the hypocercal form of tail.

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