Among the abbreviations used by systematic biologists, there is gen. et sp. nov. “new genus and species”. We have here two nouns in the singular, a neuter genus and a feminine speciēs. What would be the correct agreement of the adjective in such a case: should it be in the singular or plural, and in which gender? Did Romans use the masculine plural as the default in such mixed cases, or was the agreement tied to the first or to the last element? And let's imagine a more complicated case: ōrdō (m), familia (f), genus (n) et speciēs nov…: what do (?) / does (?) the grammar and the practice tell?