140 PHYLOGENY OF FUSUS AND ITS ALLIES. 



origin to differential acceleration, and they present parallels to the 

 varieties of C. subscalaris. Fig. i has a well-marked tuherculoides 

 stage with rounded ribless whorls which are strongly spiralled and 

 have a pronounced sutural shelf. This stage passes into the true 

 nocc stage with cylindrical whorl, strong slightly outward sloping 

 shelf, subdued spirals, and an elongated pyriform aperture with 

 a pronounced square posterior canal. The rugoidcs stage occupies 

 about four or five whorls, the last of these with faint broad ribs and 

 a strong subsutural band. The columeliar plications are scarcely 

 visible, owing to the fact that they occur far back on the columella. 

 They may be seen, however, in broken specimens. 



Fig. 2 shows a more accelerated individual in which the tuher- 

 culoides stage is almost wholly crowded out. The ribbed spire 

 (rugoidcs stage) is long, occupying about six whorls. With the dis- 

 appearance of the ribs the cylindrical form is assumed, the shelf passing 

 into the nocc stage. On this the shelf slopes strongly outward. The 

 columeliar plications are faintly visible. 



Fig. 3 shows a young specimen which has barely passed beyond the 

 rugoidcs stage. This latter is of very short duration and strongly 

 spiralled. The shelf just appears on the last ribbed whorl, there being 

 nearly a complete volution of that type. The tuherculoides stage is 

 well developed. The two columeliar plications are well shown as the 

 lip is slightly broken. It appears, furthermore, to be characteristic 

 that the columeliar plications are nearer the lip in the young and the 

 primitive species than in the adult or the accelerated species. 



In fig. 4 the shelf does not appear until after the ribs have dis- 

 appeared, thus producing a short stage comparable to the suhtuhcr- 

 culosus stage of the Claznlithcs series. This indicates that this indi- 

 vidual is less accelerated than the majority of shells of this species. 



A somewhat similar condition is shown in fig. 6 and to a very slight 

 extent in fig. 8. In the latter individual the tuherculoides stage next 

 succeeding is very short, being almost crowded out and replaced by 

 the nocE stage. In the last whorls of this stage a rather pronounced 

 projecting rim occurs which recalls the characteristics of Clavilithes 

 scalar is to the more primitive specimens of which this is a parallel. It 

 also forms a transition to R. japcti, the terminal member of this series. 



Fig. 7 shows considerable acceleration in that the sutural shelf 

 occurs in the last two ribbed whorls. Indications of the presence of 

 the two columeliar plications have been obtained in all the specimens 

 illustrated except figs. 7 and 8. The first of these is a very old indi- 

 vidual, and although the lip is broken the plications appear to be so 

 far back as not to be visible. 



In senescent individuals the inner lip is separated from the columella 

 and an umbilication is produced. In specimens where the outer lip 

 is broken away sufficiently, the columeliar plications will show, even 



