[Pteropoda] (a) Anterior part of the dorsal shell lip projecting as a snout in Thecosomata (Diacavolinia in ventral view). In some species it is clearly separated from the rest of the dorsal lip; this part of the shell hosts many discriminating characters, especially in Cavoliniidae; (b) Also used for the basal extension of the columella in coiled forms (Limacina, Peraclis) (Limacina in oral view, Peraclis in oral view). The length of the rostrum is equal to the length of the dorsal lip between rostrum tip and inner hump (in Cavoliniidae). In species without a hump the length cannot be measured exactly, though the rostrum may be present.

[Cephalopoda] The sharp, spike-like extension on the posterior tip of the gladius or cuttlebone.

[Crustacea, in general] Anterior projecting median extension of carapace between eyes or ocular peduncles; it may be straight or curved, armed with spine, teeth or being unarmed. See also spine.

[Crustacea: Ostracoda] A beak-like anterior projection of the valve margin, above and in front of the incisure; found mainly in the Myodocopida (diagram halocyprid ostracod ).

[Pisces] Any projecting snout or beak.