ioo PALEONTOLOGY 



whorls are coiled around a central cavity, the umbilicus, 

 the presence of which makes this a perforate shell. 



The aperture is nearly semicircular, the inner lip being 

 straight. In the adult animal, a fold of the mantle 

 extends out over the inner lip, and secretes an extension 

 of the inner layer of the shell, which more or less com- 

 pletely closes over the umbilicus. This deposit is called 

 callus. 



The surface of N. multipunctata is nearly smooth, being 



FIG. 28. NATICA MULTIPUNCTATA, S. V. WOOD, PLIOCENE. 



(Natural size.) 

 O.L., Outer lip ; Umb., umbilicus. (After S. V. Wood. ) 



only marked by fine lines of growth and by the puncta- 

 tions to which it owes its trivial name. In the Crag 

 specimens the lines of growth are often irregular, some 

 often several near together being more prominent 

 than the rest. As in lamellibranchs and brachiopods, 

 such irregularities denote interruptions to the steadiness 

 of growth, possibly periods of starvation or of a greatly 

 varying supply of calcareous matter. 



Species of the genus Natica are world- wide, frequent- 



