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16. 

shell imposes a natural limit.  The following table is compiled on the 1928 collection:

^[[Table I.]]
Distribution of cavities on different parts of the plastron. 

Part of plastron | Cat. No. | No of Cavities. 
Endoplastron     | 1.0.0398 |  3
  ditto          | 1.0.0426 |  2 
Epi---(left)     | 1.0.0417 |  1 
Epi---(right)    | 1.0.0417 |  1 
Hyo---(left)     | 1.0.0454 |  1
                 | 1.0.0482 |  2 
                 | 1.0.0486 |  3
                 | a gift   |  5 
Hyo---(right)    | 1.0.0395 |  2 
                 | 1.0.0418 |  4 
Hypo--(left)     | 1.0.0421 |  5 
                 | a gift   |  6 
Hypo--(right)    | 1.0.0453 |  7 
Hypoplaston(rt)  | 1.0.0517 |  5 
                 | 1.0.0458 |  1
                 | 1.0.0420 | 12 
Xiphi---(left)   | 1.0.0434 |  3 
                 | 1.0.0454 |  1 
                 | 1.0.0459 |  4 
Xiphi---(right)  | 1.0.0107 |  5 

The following notes may be appended to the above table. 

1. As the parts of the bony plate of a plastron are symmetrically arranged, the comp^[[l]]imentary parts are found usually bearing the same number of cavities. 

2. The number of cavities on the hyoplastrons is usually the same as those on the hypoplastron; sometime there is a difference