The Direction of the Thrust Produced by the Heterocercal Tails of Two Dissimilar Elasmobranchs: The Port Jackson Shark, Heterodontus Portusjacksoni (Meyer), and the Piked Dogfish, Squalus Megalops (Macleay)

1. A method of determining the location and direction of the thrust developed by the tail of a fish is described. 2. The thrust produced by the heterocercal tails of Heterodontus portusjacksoni and Squalus megalops was found to be upwardly inclined to the horizontal at approximately 12° and 26° respectively. 3. The angle of elevation of the line of thrust developed by the tails correlates with the general habits of the two elasmobranchs; particularly in regard to manoeuvrability in the vertical plane. 4. The direction of the line of thrust depends upon the positions of the leading and trailing edges of the lobes of the tail as well as the relative sizes, and general arrangement, of the lobes. 5. The ventral lobe of the tail acts primarily to reduce the angle of the elevation of the thrust. Minus the ventral lobe, both types of tail were found to be strongly epibatic with lines of thrust considerably more inclined than those of the whole tail. 6. The ventral lobe of the tail, by virtue of the radial muscles, also probably functions as a variable trim device whereby the elasmobranch can correct its ‘fore and aft trim’ by adjusting the direction of the line of thrust appropriately.