Natica or in PlanôÙorbis. It is apôÙproxôÙiôÙmateôÙly triôÙanôÙgular in Conus, and rhomôÙboidal in Solarium or PotamôÙides. It is very comôÙmonly more or less elliptical: the long axis of the elôÙlipse being parallel to the axis of the shell in Oliva and Cypraea; all but perôÙpenôÙdiôÙcuôÙlar to it in many Trochi; and oblique to it in many well-marked cases, such as StoôÙmaôÙtella, LaôÙmelôÙlaria, SiôÙgarôÙetus halôÙioôÙtoides (Fig. 284) and Haliotis. In Nautilus pomôÙpiôÙlius it is apôÙproxôÙiôÙmateôÙly a semi-ellipse, and in N. umôÙbilôÙiôÙcatus rather more than a semi-ellipse, the long axis lying in both cases perôÙpenôÙdiôÙcuôÙlar to the axis of the shell30. Its {555} form is seldom open to easy mathôÙeôÙmatôÙiôÙcal exôÙpresôÙsion, save when it is an actual circle or ellipse; but an exception to this rule may be found in certain AmôÙmoôÙnites, forôÙming the group ãCordati,ã where (as Blake points out) the curve is very nearly repôÙreôÙsentôÙed by a cardioid, whose equation is r =ã₤a(1ã₤+ã₤cosã₤ö¡).
The generating