io 4 PALAEONTOLOGY 



The surface-ornament consists of numerous slightly- 

 curved vertical ridges (corresponding to growth-lines), 

 the intervals between which are marked by finer spiral 

 lines, which become broader and flatter as they cross the 

 vertical ridges. 



Careful examination of a well-preserved specimen 

 shows two things : first, that the actual apex is obtuse, 

 and the first four whorls are quite smooth ; second, that 

 at intervals the place of about three of the vertical ridges 

 is taken by a much broader and more conspicuous eleva- 

 tion. These elevations, called varices, agree in character 

 with the thickened outer lip of the aperture ; they are in 

 fact a series of outer lips successively abandoned as the 

 shell grew larger. It is evident that a shell with an 

 elaborate aperture like that of Rimella cannot possibly 

 maintain the form of its aperture by steady growth, 

 as Natica or Turritella can do with its simple aperture. 

 It must either form its aperture once for all and abandon 

 further growth, or it must alternate between periods of 

 rapid shell-growth without the specialized aperture, and 

 periods in which no growth takes place and the elaborate 

 aperture is in full use. The latter is the case with Rimella : 

 in place of the occasional irregularities of the growth-lines 

 of Natica we have periodical growth-periods. What 

 each of these represents in actual time cannot be stated 

 with certainty, but probably less than a year. 



Another point to be noticed is that none of the varices 

 of Rimella show any sign of an old posterior canal. This 

 does not mean that posterior canals were not formed 

 until the adult stage, since immature specimens are found 



