804 THE EQUIANGULAR SPIRAL [ch. 



the shell, that is to say one on each side of a plane perpendicular 

 to the axis. In turbinate shells such a condition is also possible, 

 but is somewhat rare. We have it for instance in Solarium per- 

 spectivum, where the one contact-spiral is visible on the exterior of 



Fig. 388. Solarium perspectivum. 



the shell, and the other lies internally, winding round the open 

 cone of the umbihcus*; but this second contact-spiral is usually 

 imaginary, or concealed within the whorls of the turbinated shell. 



Fig. 389. 



Haliotis tuberculata L. ; the ormer, 

 or ear shell. 



Fig. 390. Scalaria 

 pretiosa L. ; the 

 wentletrap. From 

 Cooke's Spirals. 



Again, in Haliotis, one of the contact-spirals is non-existent, because 

 of the extreme obUquity of the plane of the generating curve. In 



* A beautiful construction: stupendum Naturae artijicium, Linnaeus. 



