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Puget Sound Marine Sta. Pub. 



Vol. 1, No. 16 



spaced." (Figs. 5-8, plate 32). "But since an injury to one mantle causes a 

 lessened vitality, even if very slight, of the entire animal, the opposite 

 mantle (shell) likewise displays a crowding and general interruption of 

 regularity in the growth lines. It thus follows that the life history of 



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PLATE 31 

 Vernier caliper for measuring specimens. 



the individual can be read from the beak forward, not only in relation 

 to the shape and size of the shell, but to its injuries, social crowdings and 

 general health." 



Consistent with this statement, the suggestion is offered that the 

 irritation or slight but constant injury, caused by the waves and currents 

 slightly bruising the tender edge of the mantle, would produce the same 

 effect all along the anterior edge of the shell. Continued indefinitely, this 

 action would result in the development of the gibbous form of the shell, 

 finally presenting a rounded front to the rough water. Altho temperature, 

 salinity, light, and various other factors may be important in the growth 

 and general health of the brachiopods, the evidence brought out by this 

 study seems to warrant the conclusion that the variations in the brachiopod 

 shells described above are due chiefly to the irritating action of rough 

 water, and are not to be attributed to any inherent tendency. 



