No. 432.] STUDIES OF GASTROPODA. 937 
which whorls will then have a larger number of varices than 
those preceding them. 
Other Ornamental Fezatures.—Dall! has demonstrated the 
purely mechanical origin of the columellar plication in gastro- 
pod shells to be due to the sliding inward and outward over the 
columellar surface of a mantle lobe folded by being crowded 
into too small a space. In Cypraa these plications are most 
numerous and developed in the adult on both sides of the aper- 
ture. The intensity of the plications varies with the nearness 
or remoteness of the muscle of fixation. In a like manner the 
liræ of the outer lip may be explained. | 
We must, however, be careful not to 
mistake the strong spirals, which some- 
times appear through the thin covering 
of the inner lip, for plications, or the 
interspiral spaces appearing prominently 
in the thin outer lip, for lire. Countess 
von Linden has discussed at length the 
development of the color pattern in gas- 
tropods,? and the reader is referred to 
her paper. 
Individual Old Age, and PAylogerontic 
Characters in Gastropods. — Gerontism, 
or old age, is marked in its earlier stages Fric. 13. — Diagram illustrating 
. . gerontic characters in the var- 
in gastropod shells by the disappear- ices of the last whorl. 
ance of features characterizing the adult. 
Such disappearance of features is generally in the reverse 
order in which they were formed. In the fusoid types the 
shoulder and the spines are the first to be lost, if they have 
been developed; or if there were only spinous tubercles, as 
in the Fusidz, these disappear together with the angulation, 
and the gerontic lip becomes smooth except for the spirals. 
The normal outline is next more or less modified by an attempt 
at straightening out the whorl and making it more cylindrical, 

1 Loc. cit., p. 58. | 
2 von Linden, Maria. Die Entwicklung der Skulptur und der Zeichnung bei 
den Gehiuseschnecken des Meers, Zeitschr. für wissensch. Zoologie, Bd. lxi, 
pp. 261-317. 1 pl. 
