Momenta Pharmaceuticals, Inc. (MNTA)

[semi_infinite]

Most probably a call spread. I think the higher probability is that it was initiated on the buy side where someone is buying the 12 calls and selling the 13 calls for a net cost of 0.27 per contract.
The alternative view would be that someone took advantage of the positive move yesterday to do a bearish spread, but I don't think so as the risks doesn't favor that. The max profit would be 0.73 if a positive announcement happens before Dec expiration. Even if, nothing happens by Dec expiration, the spread will probably have some value and the party can exit out of the position with less than max loss of 0.27.