Spirals: their orientation and description

Abstract. The trochoid coiling mode is the most dominant amongst hyaline foraminifera. The parallels in shape with especially gastropods neatly explains the use of the same terms to describe the spiral coiling. Other terms came in use, at first well-defined, but gradually losing their precision. Because of growing demands of systematics and changes in stress of use and value of characters, the inadequacy of this terminology has become a stumbling block. The terms ventral/dorsal and spiral/umbilical denote different things and are therefore all useful: the latter pair should not be relinquished as suggested (Haynes, 1990: 512). Historically, the terms ventral/dorsal and upper and lower side have been used by most students of the foraminifera with only few exceptions (notably Reuss and Loeblich & Tappan). Despite the protist nature of foraminifera, dorsal and ventral continued to be used, analogous to terms used to describe, for example gastropods. Far fewer problems beset the terms spiral and umbilical, which arc generally defined (Oxford English Dictionary) as: spiral a.. n. & v. 1. a. Coiled; winding about a centre in an enlarging or decreasing circular motion, either on a flat plane or rising in a cone. spire n. Spiral, coil; single twist of this; upper part of spiral shell [F. f. Lf. Gk speira coil] umbilical a. 1. Of, situated near, affecting, the umbilicus 2. Centrally placed. umbilicus n. Navel; (Bot. & Zool.) navel-like formation; (Geom.) point in a surface through which all cross-sections have same curvature [I., rel. to Gk omphalos]

ORIENTATION: A GEOMETRICAL . . .