the anterior part of the dorsal lip projecting as a snout before the shell in Thecosomata. In some species it is well separated from the rest of the dorsal lip; in this part of the shell many discriminating characters are located, especially in Cavoliniidae.
The term rostrum is also used for the prolongation of the columellar structure in coiled forms (Limacina, Peraclis).
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 is present.