352 



PROCEEDINGS OF THE ACADEMY OF 



[May, 



9. Tendency to jorm a smooth and rounded whorl.- — In many gastro- 

 pods the later whorls tend to become rounded and more or less smooth, 

 losing all ornamental features except growth lines and perhaps also 

 fine spirals. In many individuals and varieties of Fulgur 'pyrum and 

 related forms the shoulder keel stage is followed by a condition in which 

 the whorl is roimded, the shoulder keel becoming fainter and fainter 

 vmtil it is not discernible. In many gastropods this rounded stage is 

 accompanied by a thickening of the shell, but in others the shell actu- 

 ally becomes thinner. As the early whorls of most gastropods are 

 rounded, it is possible that this mark of senility may be regarded as 

 a reciu-rence of features of the same nature as the mark of senility next 

 to be considered. 



Fig. 1. — Fulgur maximum Con. Mio-' 

 uene. Cape Fear River, N. C. Diameter 

 of shell across last whorl = 61 mm. Api- 

 cal view exhibiting recurrent or discon- 

 tinuous ornamentation. 



Fig. 2. — Fidgur maximum Con. 

 Miocene. Cape Fear River, N. C. 

 Diameter of shell across last whorl 

 = 52 mm. Apical view exhibiting 

 recurrent or discontinuous orna- 

 mentation. 



10. Recurrent or discontinuous ornamentation. — -This is a term which 

 may be applied to the discontinuity produced by the repetition, at a 

 later stage, of a style of ornamentation characterizing an earlier stage. 

 Thus spines or tubercles which are found on the early whorls disappear 

 for a few whorls, but are repeated as a secondary series in old age. 

 The development of secondary spines is well shown in Fulgur maximum. 



In most examples of this very variable species and its near relatives 

 the shoulder angle tubercles are soon lost, and only reappear again in 

 the later whorls, where they are spine-like and accompanied by features 

 of extreme senility. Sometimes the arrangement of the shoulder angle 

 spines may be considered as discontinuous, that is, where an occasional 



