No. 432.] STUDIES OF GASTROPODA. 931 
whorls are, as already suggested, simple ribs, or spirals, or both. 
When the whorls remain round throughout the life of the indi- 
vidual no other regular ornamentation is as a rule produced, 
except that in the final (gerontic) portion of the last whorl the 
ribs, and in some cases the spirals, may disappear, leaving the 
shell smooth. If, however, a shoulder is produced through 
the formation of an angulation, a new succession of ornamental 
types will arise, which generally appears in the same sequence 
in widely separated groups of gastropods. The first effect of 
the angulation seems to be to concentrate the growth force on 
it. The ribs become accentuated on the angle, and correspond- 
: ingly fainter away from it. Finally, they disappear above and 
below the angle, being represented on the latter, however, as 
a series of tubercles. These tubercles, though generally blunt, 
may at times assume a somewhat spinous form ; but they appear 
to be distinct from true spines, which, so far as observed, 
are a feature of later growth. In the cases in which actual 
succession has been traced out in series (Fulgur, Fusus, Hemi- 
fusus, etc.) it was found that the knobbed or tuberculated stage 
is succeeded by a stage in which the tubercles become con- 
fluent, producing a pronounced ridge or keel on the angulation. 
This after a time disappears, and with it the angulation, the 
outline of the whorl becoming rounded again, with only the 
spirals persisting. These in some cases may disappear also, 
or at least become very faint. After the whorl has returned 
to its primitive rounded outline a new type of modifying ele- 
ment arises in the form of spines. These begin as notches in 
the margin, generally in the spiral zone of the angulation of 
the earlier whorls, but sometimes in a second or even a third 
zone lower down on the body of the whorl (e.g. Melongena). 
They mark a distinct and periodic fold in the outer lobe of 
the mantle, which sometimes is accompanied by a temporary 
cessation in shell building, as indicated by more or less strongly 
marked resting lines in the lines of growth of the shell (Fulgur 
carica, etc.). They thus have all the characters of varices of 
ounced in Murex, where their multiple devel- 
the type so pron 
the most 
opment and frequent compound character produce 
striking feature of the group of shells generally associated 
