XI 



OF CERTAIN OPERCULA 



775 



in (e.g.) Nerita (Fig. 371); that is to say, apart from the side con- 

 stituting the outer edge of the operculum (which side is always and 

 of necessity curved) the successive increments constitute curvihnear 



Fig. 369. Operculum of Turbo. 



triangles in the one case, and rectilinear triangles in the other. 

 The sides of these triangles are tangents to the spiral Une of the 



Fig. 370. Fig. 371. 



Figs. 370, 371. Opercula of Turbo and Nerita. After Moseley. 



operculum, and may be supposed to generate it by their consecutive 

 intersections. 



In a number of such opercula, Moseley measured the breadths 



