NEED TO CLASSIFY—BATTEN 515 
Figure 5.—Growth of rock-clinger, from early whorl to final cap-shaped form, is seen here 
with Fissurella, the keyhole limpet, another large limpet group belonging to the super- 
family Fissurellacea of the Archeogastropoda. First three growth stages are shown 
greatly magnified. 
organisms shared a similar environment, they were vastly different 
from each other. What was not so apparent, however, was that some 
of the gastropod groups that possessed cap-shaped shells were very 
different from other gastropod groups that also possessed such shells. 
A few years ago, paleontologists attempted to reclassify some cap- 
shaped fossil shells that dated back to the early Paleozoic. With 
no soft parts to examine directly, they critically studied these shells 
for characteristics that could be related to the absent tissue. In the 
study of living patellids, it had been noted that a continuous “muscle 
scar” ran around the inside of their shells, marking the attachment 
area for the limpets’ powerful muscles. The paleontologists found 
that many of the fossil shells exhibited such a continuous “muscle 
sear.” Therefore, they felt safe in assuming that the missing soft 
parts had been similar to those found in living patellids. 
However, the paleontologists also discovered that other cap-shaped 
shells possessed, instead, two to eight pairs of distinct “muscle scars.” 
Two things were curious about these ancient muscle scars: first, some 
were eight in number and, second, they were arranged around the 
shell in bilaterally symmetrical pairs. The paleontologists could 
only speculate that these shells were, in fact, so primitive that the 
