[538]. (b.) Stems in -ri-, with nominative -s of the stem, genitive -ris, are glīs, F., dormouse, glīris; mās, M., male, maris; mūs, F., mouse, mūris.
[539]. The only imparisyllabic stem in -si- is ās ([171, 1]), M., unit, an as, G. assis, with its compounds bēs, two thirds, G. bessis, and sēmis, half an as, half, G. sēmissis.
SINGULAR CASES.
[540]. (1.) The nominative singular of gender stems in -i- is usually formed by adding -s to the stem ([422]). But many gender substantives have the nominative in -ēs ([236], [401]): as,
amni-, river, N. amnis; aedi-, temple, N. aedis; brevi-, short, N. brevis. With N. -ēs: nūbi-, cloud, N. nūbēs; for other examples, see [523].
[541]. Some substantives form the nominative in both these ways: as, vallēs and vallis, valley, equally common; aedis, temple, later aedēs; for caedēs, slaughter, clādēs, disaster, and mōlēs, pile, caedis, &c., occur exceptionally.
[542]. A few stems in -bri-, -cri-, or -tri-, drop -i- in the nominative. The endings brs, crs, trs, then change to -ber, -cer, -ter ([111, b]): as, imbrī-, shower, N. imber ([525]).
[543]. Of gender imparisyllables, some have lost -i- of the stem before -s in the nominative; others have originally a consonant stem in the nominative ([529-535]).
Thus, monti-, mountain, and sorti-, lot, have N. mōns and sors for an older montis and sortis; but dēns, tooth, and regēns, ruling, have as original stems dent- and regent-. Adjectives in -cipiti- have N. -ceps ([533]).
[544]. A few adjective stems in -li- or -ri- drop -i- in the nominative without taking -s ([536], [537]): as, vigili-, wide-awake, N. vigil; pari-, equal, N. pār; so also Arar and Liger. Three substantives in -ri- for -si- likewise drop -i-, and end in the original -s ([538]): glīri- for glīsi-, dormouse, N. glīs; mās, male; mūs, mouse.