Some geometrical aspects of fusiform planispiral shape in larger foraminifera

Abstract. Fusiform planispiral tests are found in many lineages of larger foraminifera (e.g. fusulinids, lituolids, alveolinids, soritids, amphisteginids) in which a general trend from spherical or discoidal to elongate test shape has often been noted. Geometrical models are used to illustrate an increasing surface area and the shortened spiral lines of communication of such a trend, with whorl and chamber proportions and volume or spiral length held constant. The suggestion that fusiform planispiral growth is a device to shorten lines of communication from the proloculus to the poles is questioned.