Lanceolate refers to a "leaf shape" and may range from wide to narrow or fusiform.
Barber, H.G. and Hayworth, E.Y. (1981). Guide to the Morphology of the Diatom Frustule.
Freshwater Biological Association, 112 pp.
Image Credit: Marina Potapova
The valve margins of Navicula tripunctata are narrow lanceolate.