very fact of its being a spiral soon ceases to be apparent (Figs. 271, 272). Suppose one whorl to be an inch in breadth, then, if the angle of the spiral were 80ô¯, the {535} next whorl would (as we have just seen) be about three inches broad; if it were 70ô¯, the next whorl would be nearly ten inches, and if it were 60ô¯, the next whorl would be nearly four feet broad. If the angle were 28ô¯, the next whorl would be a mile and a half in breadth; and if it were 17ô¯, the next would be some 15,000 miles broad.
In other words, the spiral shells of gentle curvature, or of small constant angle, such as Dentalium or Nodosaria, are true logarithmic spirals, just as are those of Nautilus or Rotalia: from which they differ only in degree, in the magnitude of an angular constant. But this diminished magnitude of the angle causes