In Souriau's book of symplectic mechanics he describes an elementary dynamical system on which the Poincare group is dynamic and acts transitively. He then describes a massive particle with spin where the spin is some positive number. When we consider this particle in the presence of an external electromagnetic field then this spin has similar properties to that considered in QM, such as the elementary particle has a magnetic moment parallel to the spin. I asked my lecturer if it was then the case that spin is a property of relativity rather than QM but he has not read Souria's book but said that he had heard that Souriau had his own intepretation of spin. I was then wondering if anyone knew of the intepretation of this intrinsic spin described by Souriau, if the system is quantized will this correspond to spin in QM?