SHELES WITHOUT -CANALS 23: 
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Figure 256 shows the shell of 
a large Acmea persona, Ksch., 
the Mask Limpet. This shell 
may be distinguished by the 
position of the apex, which is situated very near 
one end, making nearly all the slope come upon 
one side, like the roof of an old-fashioned farm- 
house. The ribs on the slope of the shell are 
prominent but irregular. The outside is gray or 
mottled, and the inside has varying amounts of 
brown and white. The shell is high arched, but 
seldom grows to the length of an inch. This 
species is generally found living where the rocks 
are seldom covered with water. 
Acmea scabra, Nutt., the File Limpet, is usually 
of a light brown color externally and white inside. 
Sometimes the brown is so light that it is almost 
yellow, while again the surface may be quite dark. 
The arch of the shell is generally low, and there 
are fine, sharp, scaly ridges radiating from the 
apex, making the shell feel like the surface of a 
fine-cut file. Sometimes it is a little hard to tell 
a scabra from a patina, just from the shell, but 
if you ean see the animal you ean at once decide, 
for the head and mantle of the former are black, 
while those of the Plate Limpet are always white. 
The ordinary length is about an inch, and the 
arch of the shell is commonly quite low and the 
shell thin. Occasionally very aged specimens are 
found, which have lost all their seulpture, and 
have become very thick. They can be told, how- 
ever, by the white appearance of the interior of 
