XI] 



OF SHELLS GENERALLY 



805 



Scalaria pretiosa and in Spirula* there is no contact-spiral, because 

 the growth of the generating curve has been too slow in comparison 

 with the vector rotation of its plane. In Argonauta and in Cypraea 

 there is no contact-spiral, because the growth of the generating 

 curve has been too quick. Nor, of course, is there any contact- 

 spiral in Patella or in Dentalium, because the angle a is too small 



Fig. 392. Turhinella napus 

 Lam.; an Indian chank- 

 shell. From Chenu. 



Fig. 391. Thatcheria mirabilis Angas; 

 from a radiograph by Dr A. Miiller. 



ever to give us a complete revolution of the spire. Thatcheria 

 mirabilis is a peculiar and beautiful shell, in which the outhne of 

 the Up is sharply triangular, instead of being a smooth curve : with 

 the result that the apex of the triangle forms a conspicuous "gene- 

 rating spiral", which winds round the shell and is more conspicuous 

 than the suture itself. 



In the great majority of hehcoid or turbinate shells the innermost 



* "It [Spirula] is curved so as its roundness is kept, and the Parts do not touch 

 one another": R. Hooke, Posthumoics Works, 1745, p. 284. 



