THE GASTROPODA 99 



united into a solid pillar extending from base to apex 



the columella. 



The outline of the whorls and detail of the surface- 

 ornament are specific characters of T. imbricataria .- all 

 the other characters described (except where common to 

 all or many gastropods) are generic characters of 

 Turritella. A shell of this shape, with long spire, and 

 last whorl not conspicuously larger than the last but one, 

 is described as turreted. The possession of a solid 

 columella is denoted by the term imperfoyate ; the simple 

 outline of the peristome is indicated by the term holosto- 

 matotts. These terms will be more clearly understood 

 when the contrary terms are illustrated. 



3. Natica multipunctata (Fig. 28) is a common fossil 

 in the masses of accumulated shells found in East Anglia 

 and known as Crag, and it still lives at the present day, 

 though only in warmer seas, as the Mediterranean. The 

 shape is very unlike that of Turritella, the last whorl 

 forming the greater part of the external surface, the spire 

 being very low, and having a spiral angle of 120. The 

 whorls are highly convex, nearly semicircular in outline, 

 but flattened or even slightly concave near the suture. 

 The large size of the last whorl is not due to an abrupt 

 increase of sectional area, but to the mode of coiling, the 

 greater part of each whorl being concealed by the next : 

 this can be realized either by following in imagination the 

 extension of the last whorl which would result from 

 further growth, or by making an actual section through 

 the vertical axis. Such a section would also show that 

 instead of being united centrally into a columella the 



