256 



SCULPTURE AND ORNAMENTATION 



CHAP. 



The deposition of shell is not continuous. Rest 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 

 growth. In some cases {JSIurex, Triton^ Ranella)^ the rest period 

 is marked by a decisive thickening of the lip, which persists on 

 the surface of the shell as what is called a varix (see p. 263). 



The various details of sculpture on the exterior surface of 

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

 forms of ornamentation are all the product of similar and corre- 



FiG. Ifio. — Neritina longi- 

 spina Reel., Mauritius. 

 (Operculum removed.) 



Ftg. 164. — Murex tenuispina L., Ceylon. 



sponding irregularities in the mantle margin, and have all been 

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

 Murex and Pteroceras^ are first formed as a hollow thorn, cleft 

 down 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 spiny processes in some 

 cases, can hardly be considered as satisfactorily ascertained. 

 Possibly they are a form of sculptural development which is, in 



