in space generated by any given point partakes, therefore, of the character of a helix, as well as of a logarithmic spiral; it may be strictly entitled a helico-spiral. Such turbinate or helico-spiral shells include the snail, the periwinkle and all the common typical Gastropods.
The generating figure, as represented by the mouth of the shell, is sometimes a plane curve, of simple form; in other and more numerous cases, it becomes more complicated in form and its boundaries do not lie in one plane: but in such cases as these we
may replace it by its ãtrace,ã on a plane at some definite angle to the direction of growth, for instance by its form as it appears in a section through the axis of the heliôÙcoid shell. The genôÙerôÙating curve is of very various shapes. It is circular in Scalaria or Cyclostoma, and in Spirula; it may be conôÙsiôÙdered as a segôÙment of a circle in