614 THE AMERICAN NATURALIST [\^ol. XLI 



ceding whorl at yl (100%). O — B represents progressive loss 

 of coiling power to straight conditions at B; — C, increasing 

 horizontal ity of shoulder from round at to 45° and thence to 

 horizontal or 90° with reference to the axis of coiling. — D 

 in the same way represents increasing verticality from 45° to 0°, 

 or parallelism with reference to the axis of coiling, at D. 



A simple round whorled shell in w^hich the whorls do not em- 

 brace throughout Hfe (primitive Fusus) w^ould be represented by 

 the point 0. One in which a slight amount of embracing occurs, 

 but no change in outline of whorl, is represented by a line on O — A, 

 the length of which marks the percentage of embracing up to 50 

 at the ambitus or more if the whorls overlap. The line may not 

 start at but higher up, the earlier whorls showing some embrac- 

 ing. The line — a represents the life of a shell in which the 

 whorls gradually change to angular (45°) and the embracing 

 increases up to 25% or half w^ay to the ambitus. 



— h represents a shell with gradual increase of embracing 

 of whorls to ambitus, and flattening to 75° with reference to the 

 axis of coiling, as in some Fulgurs. — c represents a more 

 rapid flattening than increase in embracing, as in Conus; — d 

 a gradual increase in embracing to below the ambitus, while at the 

 same time the whorls are flattened to a shoulder of 45°. Then 

 the embracing continues to the ambitus, beyond which, w^ith in- 

 creasing embracing, the whorls lose their angularity. This is seen 

 in Melongena. In Clavilithes, represented by — /, embracing 

 increases to the ambitus, after which the whorls become quickly 

 flattened vertically to 0°. Vermetus is represented by — g, in 

 which increasing embracing and flattening to 10° or less represents 

 the Turritella stage, after which a loss of the embracing finally brings 

 us to the loose coiling type with a return to round whorled condi- 

 tion. Similar changes are seen in Trohonema vagrans (Studies 

 I, fig. 15) where angular embracing whorls are succeeded by loose 

 rounded ones. 



Ornamentation of the Whorls. 



. As already shown in the first of these studies (.1 m. Nat., XXX\T, 

 p. 930) the ornamentation of the whorl consists of ribs, spirals^ 



