NEED TO CLASSIFY—BATTEN 519 
Bch 
yy 
ary 
Se 
\ 44 
Ficure 9.—Top view of chiton shows eight separate plates of its shell. 
be so closely related as the revised classification proposed. It was 
necessary, therefore, to revise the classification further, so that the 
monoplacophorans were placed in with the chitons, while the gastro- 
pods were viewed as a branching-off from the chiton stock. 
Again, a major change in classification had been made, in order 
to fit newly discovered facts (fig. 8). 
Ficure 10.—Multiple muscle scars of fossil monoplacophorans (left) are contrasted here 
with the continuous, ringlike muscle scar of a fossil patellid. With this clue to bilateral 
symmetry, the paleontologists reconstructed a hypothetical set of symmetrical body organs 
for the monoplacophorans (right) in which anus and mouth lie at opposite ends of the 
animal. 
