[Gastropoda general] The angle between the lateral shell sides enclosing the embryonic shell in profile view. In some species two angles are given when the top angle changes by the loss of an embryonic shell like in Diacria or when in the adults the spines create a larger top angle as in Cavolinia.

[Pteropoda] The angle between the lateral shell sides enclosing the embryonic shell, in side view. Peraclis in oral view

Alternative form for Apex angle : top angle.