284 Introduction to Animal Morphology. 



however, the generating figure as it proceeds slips 

 down out of the original plane along a vertical axis, 

 and the angle of descent is usually a constant one, 

 and contributes another element to those necessary to 

 be known to determine the shell form. This may be 

 called the helicoidal co-efficient, and as its value for 

 the last few whorls in multispiral shells may differ 

 from its value for the earlier whorls, shells may be 

 diplohelicoid as well as diplospiral.* 



The shell consists of several strata of prisms, which 

 vary in inclinations. It is lined by a smooth layer, 

 not often nacreous. The mineral matter being biaxial 

 in its behaviour to light, resembles Arragonite ; it 

 forms 82-99 P er cent - of the shell, and gives it its 

 LC gravity (2.70-2.97) and hardness. The 

 animal matter is conchiolin, and forms 0.82 - 8. per 

 cent. In ( \ prgea, Turbo, &c., the shell is porcellanous, 

 and in the former the mantle lobrs are reflected over 

 the outside of the shell, on which they secrete an 

 enamel lavr. Some shells contain phosphoric acid, 

 magnesia, and iron oxide. ('. Schmidt found the 

 superficial mantle cells to contain a calcic albuminate, 

 from which the shell substance is doubtless laid down. 

 The periostracum (absent in Cypraea, &c.) may be 

 pilose, silky, lamellar, or shaggy. 



In coiled shells the smooth and porcellanous apex 

 often differs in appearance from the succeeding 

 whorls; it consists of the original shell of the embryo, 



* The following clarification includes all shell forms : i. Spiral, in- 

 cluding a, turretcd ; ,-?, cylindrical ; y, short ; c. globular J depressed ; 

 , discoidal; 17, convolute ; 0, trochiform ; i, fusiform ; K, turbinate; A, pau- 

 ci.-piral ; /i, multispiral ; v, ear-shaped. 2. Irregularly spiral. 3. Shield- 

 shape. 4. Boat-shaped. 5. Conical. 6. Multivalve (Chiton). 7. Pseu- 

 docamerated (Euomphalus, Triton (Simpulum), corrugatum). 



