Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sembella, ae, f. [contr. from semi and libella], a half-libella (a coin): sembella quod libellae dimidium, quod semis assis, Varr. L. L. 5, § 174 Müll.: eandem rationem habere assem ad semissem, quam habet in argento libella ad sembellam, id. ib. 10, § 38 ib.