You will receive one share of Prothena for every 41 shares of Elan. So if you own 1000 shares of Elan you will receive 24 shares of PRTA. Also since the settlement date was Dec 14th you would have to have bought Elan no later then Dec 11th to qualify for free shares.
i know nothing, but simple logic tells me that it should be 2 scenarios: first - new (demerged) company's share should be given one for one with the share price being calculated as sum of current and future(speculative) input. tysabri's part share price will be obviously changed(including options); second,number of shares of new company will be distributed as a fraction of total number of shares( i wouldn't expect more than 20-25%).perhaps,there is third scenario when we will loose part of what we own.