There seems to be quite a lot of confusion about the name of this fragrant lily. You'll sometimes seem it listed as Ismene x festalis, spider lily, Peruvian daffodil and also Peruvian lily. There are over 60 species of Hymenocallis, that come from South America and the southern United States. This example is a hybrid (denoted by the x between the generic and specific names) between H. longipetala and H. narcissiflora and is notable for its long stamens and very long style. When a pollen grain lands on the stigma and germinates the pollen tube that it produces has to grow over 10cm. through that style to reach and fertilise the ovules - a remarkable length for what is effectively a single cell. I've only grown this bulbous plant once but it thrived in a cool greenhouse.
Andrea has posted a lovely photograph of another species, Hymenocallis littoralis, on her blog.