lOO PHYLOGENY OF FUSUS AND ITS ALLIES. 



ribs of the conch. The protoconch of this genus, therefore, combines 

 the characters of Fiisiis and Clamlithcs, and this, together with the 

 characters of the conch, indicates that Clavellofusus occupies a posi- 

 tion intermediate between Fiisiis and Clavilithes. 



The conch begins with nearly uniformly rounded whorls, with 

 equal and uniform ribs, which, however, quickly become swollen in the 

 center, and then die out towards the suture. They are widely separate 

 and crossed by uniform spirals. In some cases the ribs are separated 

 by interspaces equal to three or four times their own width, and the 

 spirals generally show a perfect gradation in thickness and in spacing, 

 from the periphery to the sutures. Intercalated spirals appear in about 

 the fifth or sixth volution. 



All the whorls of the early stage are globular, and embrace about 

 a fourth of the preceding whorl. The spire is, therefore, much more 

 elongated and slender than in species of Clavilithes. 



In the early neanic the ribs change from a definite sharp and dis- 

 tinct outline to an ill-defined one, becoming more of the nature of 

 undulatory wrinkles. In the later neanic they become obsolete, as 

 do also the spirals. Intercalation begins in the late neanic, seldom 

 earlier. 



The ephebic whorls are globular at first, and free from ribs or 

 spirals. A very faint shelf is developed next to the suture which grad- 

 ually grows broader. This shelf is caused by the development of a 

 deep and strong posterior canal, and it is accentuated by the slight con- 

 vexity of the later whorls, by their strong embracing of the preceding 

 whorls up to or beyond the middle and by the consequent slight depres- 

 sion of the suture. 



In the met-ephebic stage the whorls have changed from a rounded 

 to a cylindrical form, the sides of the whorl becoming parallel to the 

 longitudinal axis of the shell. This gives the shell a rigid appearance 

 which is very marked. The sutural shelf is very pronounced at this 

 stage. It is sharply delimited at the margin, and makes a right angle 

 or something less with the outer face of the shell. It slopes slightly 

 inward from the sharp shoulder angle. This species may be compared 

 with accelerated individuals of Clavilithes parisiensis, in which the 

 sutural shelf appears in the conjiinctus stage. Except for the long 

 spire, it could be considered a parallel to Clavilithes siibscalaris. The 

 distinctive generic characters allow, however, ready separation. 



Injured and gerontic individuals show a decrease in the size of the 

 aperture and a loss of the characteristic sutural terrace or shelf. In 

 such individuals the inner lip also separates from the columella, thus 

 showing a contraction of the aperture on all sides. 



The specimen illustrated in fig. 20, pi. I, is somewhat more ac- 

 celerated than the typical forms. The stage with round whorls and 



