No. 432.] STUDIES OF GASTROPODA. 943 
spines (of stage 7) are.very vigorous. Thus it will happen, 
as in H, that stages 5 and 6 are dropped out altogether, 
stage 7, the spinous one, following upon stage 4, the tubercled 

Fics. 17 and 17 a. — Cyr£ulus serotinus Hinds. Recent. A phylogerontic type of 
the Fusus series. Natural size. 

Fic. 18. — Clavilithes chamberlaini Johnson and Grabau. Eocene. Texas. Two stages; the 
adult showing a serrated flange and clavilithoid form. (Proc. Phil. Acad. Nat. Sci., 
mber, 19or.) 
one, and even encroaching upon it. This is acceleration by 
elimination (example: Fulgur carica). Finally, even the vigorous 
Spines disappear, and the final smooth gerontic stage is formed. 
