Trithrinax refers to the presence of not just one Thrinax, but three.  Actually, it is a genus of at least 4 species of very spiny fan palms from South America.  Not only are they very spiny; they are also just all around tough and resistant to cold, heat, wind, drought and other adverse conditions.  T. campestris and T. schizophylla are the hardiest species.  They will probably grow in the milder parts of the Pacific Northwest, given very good drainage, but they are so incredibly slow-growing and spiny, it is doubtful they will ever become popular.  T. acanthocoma (syn. T. brasiliensis) is faster growing and easier to obtain, but it is not as cold hardy and often freezes at temperatures in the vicinity of -7°C/19°F.  T. campestris might even be worth trying east of the Cascades, where the extra summer heat would be advantageous.  Some have claimed T. campestris to be the world's most cold-hardy palm; although that is probably an exaggeration, its limits are not really known and it can certainly withstand temperatures down to -17°C/0°F once established.