256 



SCULrTURE AND ORNAMENTATION 



The deposition of shell is not continuous. Eest periods occur, 

 during which the function is dormant ; these periods are marked 

 off on the edge of the shell, and are known as lines of grovjth. 

 in some cases {Murex, Triton, lianella), the rest period is marked 

 hy a decisive thickening of the lip, which persists on the surface 

 of the shell as what is called a varix (see p. 26-')). 



The various details of sculpture on the exterior surface of the 

 shell, the striae, rihs, nodules, imbrications, spines, and other 

 forms of ornamentation arc all the product of similar and 



Fiu. 165. — Neritina lonf/i- 

 spina Rt'c:l. Mauritius. 

 (Opei'culuiu removed.) 



Vm. 104, 



Ulicrex tenvispina L., Ceylou. 

 X3- 



corresponding irregularities in the mantle margin, and have all 

 been originally situated at the edge of the lip. Spines, e.g. those 

 of 3furex and Fteroceras, are first formed as a hollow thorn, cleft 

 dcnvu its lower side, and are afterwards filled in with solid matter 

 as the mantle edge withdraws. What purpose is served by the 

 extreme elaboration of these spiuy processes in some cases, can 

 hardly be considered as satisftictorily ascertained. Possibly they 

 are a form of sculptural development which is, in the main, pro- 



