4 PHYLOGENY OF FUSUS AND ITS ALLIES. 



conch. Thus acceleration is indicated in the protoconch of the Fusidse 

 by a pushing back and appearance upon its whorls of features nor- 

 mally characteristic of the earliest or nepionic stage of the conch. 



The degree of acceleration is indicated in the successive members 

 of a group, by the age at which distinctive features appear or dis- 

 appear. Such features are of different types in different groups, but 

 they may be classed under form and ornamentation of the whorls. 

 In general, the more accelerated a fusoid shell is, the earlier the change 

 from rounded to angular whorls is accomplished, and the sooner inter- 

 calated spirals make their appearance. This is progressive acceleration. 

 In regressive types, acceleration is indicated by the early disappearance 

 of ribs, of the peripheral angulation, and finally of the spirals. 



The spirals or revolving striae are of especial significance, as indi- 

 cative of the degree of acceleration which the individual has attained. 

 They may be divided into two groups, primary and post-primary. 

 The first group makes its appearance more or less abruptly, especially 

 in Fiisns, where three or four spirals suddenly appear at the beginning 

 of the nepionic stage. In other gastropods, these spirals may begin 

 as a single one, increasing by the appearance of new ones on either 

 side. This method of increase may be designated as exogenous, the 

 new spirals appearing outside of the older ones ; that is, between the 

 sutures and the spiral next to it.* The second group has an endogen- 

 ous manner of growth, the new spirals always becoming intercalated 

 between the older ones. Secondary spirals appear between the primary 

 ones, and tertiary between the primary and secondary. Sometimes 

 spirals of a fourth or fifth and even of a higher cycle appear. From 

 the method of appearance in the individual shell we can formulate the 

 general law that species of a group in which primary spirals alone 

 exist, are more primitive than those in which spirals of a higher cycle 

 occur; and that in general, the higher the cycle of intercalated spirals, 

 the more specialized or accelerated has the individual become. 



Another feature by which we can measure the degree of accelera- 

 tion is the posterior canal, and the accompanying subsutural band or, 

 in some cases, the sutural shelf or terrace. The posterior canal is of 

 the nature of a notch, at the point where the outer lip joins the body- 

 whorl. This notch, weakly or not at all developed in the adult of 

 primitive species, appears earlier and earlier in the successively accel- 

 erated types, and becomes more and more pronounced in the adult. 



* It must be borne in mind that only below the upper suture on any whorl can 

 we see the true edge of the shell, except on the body whorl. New spirals will 

 therefore be seen next to the upper suture only, as we trace their development in an 

 adult shell, while the new spirals which appear on the lower border of the whorl 

 — i. c, what was once the spindle of the body-whorl — are covered up by the suc- 

 ceeding whorls except in the case of the very last one. 



