The spelling is determined by the published name accepted, independent of how the person after whom the genus is named spelled his name. It happens all the time. In this case, the matter was cleared up a long time ago. On page 284, para. (d) of Botanical Latin (fourth edition, 1992) by William T. Stearn, we read "Lapeirousia and Peyrousea , both after P. Picot de La Peyrouse." Andrew San Diego