1. V. Bargmann:Zeits. f. Phys.,99, 576 (1936).
2. J. Jauch andE. Hill:Phys. Rev.,57, 641 (1940).
3. A. Sàenz (Thesis, Michigan University, 1949) was the first to discuss in a comprehensive way the general problem of dynamical symmetries in classical mechanics; we are grateful to Dr.Sàenz for making a copy of his Thesis available to us.
4. J. Fradkin: Wayne University preprint (1966);
5. M. Han andP. Stehle: Pittsburgh University preprint (1966);