Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mĕtella, ae, f. (prob. adj., sc. machina), a basket filled with stones, which the besieged threw down on the heads of the besiegers (post-class.): ut de ligno crates facerent, quas metellas vocaverunt, lapidibusque complerent, etc. (al. metilas, medullas, metulas), Veg. Mil. 4, 6.