Paeninsulis is either the dative plural or the ablative plural of
paeninsula, -
ae, f., "peninsula". The full declension is:
Singular
- Nominative/vocative: paeninsula
- Genitive: paeninsulae
- Dative: paeninsulae
- Accusative: paeninsulam
- Ablative: paeninsula
Plural
- Nominative/vocative: paeninsulae
- Genitive: paeninsularum
- Dative: paeninsulis
- Accusative: paeninsulas
- Ablative: paeninsulis