Genitive case forms in Latin are peculiar to each declension. In fact, the way a noun forms its genitive is what determines the declension to which it belongs. The dictionary listing for a noun gives the nominative, genitive, gender, and meaning (e.g., os, ossis, n. — bone). The nominative form, os, has to be provided because it cannot always be predicted. The second word, ossis, is the genitive. From it you learn the stem change (oss-). The genitive ending -is tells you that it is third declension.

Table 9-2 Genitive Case Forms