936 THE AMERICAN NATURALIST.  [Vor. XXXVI. 
In Scalaria the varices are simple labial flanges, which stand 
out as sharp ridges, with the appearance of ribs. In He/ix 
albolabris and other types the varix is formed only in the 
adult. In all types with successive varices it may be observed 
that the varices of the same longitudinal 
zone do not form a line parallel to the 
axis of the shell but that this line runs 
spirally backward, in the direction opposite 
to that in which the whorls coil (Fig. 12). 
This is due to the fact that there is a 
progressive diminution in the proportional 
amount of building which was performed 
between the resting stages. If theamount 
of building were proportional to the size 
aide EU of the shell the varices would run in 
with " proportional Straight, constantly diverging lines (Fig. 
TET II) instead of spiral ones. If the propor- 
tional increase were too great for the size of the shell, and 
progressively increasing, the lines of varices formed by the 
corresponding ones of all the whorls would pass spirally forward, 
l.e., iv the direction of coiling of the shell Neither this nor 
the case of increase proportional to the 
size of the shell has been noted, and it 
hence appears that we have here an 
illustration of Minot's law of senescence, 
the animal progressively growing old from 
the beginning. This feature is best shown 
in Scalaria, where in large specimens an 
additional feature, first pointed out to the 
writer by Prof. R. T. Jackson, is also seen. 
This is the more rapid shortening of the 
building stages, in the old age of the indi- Fic. 12.— Diagram 
vidual, so that the varices of the last— — teristic retard ed growth 
old age — whorl are not continuous with 
those of the preceding ones but fall between them, so that 
sometimes a varix may be the whole length of a period 
behind what it should be (Fig. 13). In phylogerontic species 
of Scalaria this feature appears in the adult or earlier whorls, 

