Evans: Air chambers of Grimaldia fragrans 251 



The increase in the size of the chambers is due largely to the 

 growth of the bounding cells and only slightly to further splittings 

 of cell walls. The system of united cell plates in the dorsal cham- 

 bers and the partitions between the chambers increase in vertical 

 height simultaneously. Direct outgrowths from the surfaces of 

 cell plates play a very small part in the process of subdivision. 



The material upon which this investigation was based was col- 

 lected and prepared by Mr. John F. Logan, who expected to 

 utilize it in his own studies. Through the pressure of other work 

 his plans could not be realized, and his preparations were placed 

 at the disposal of the writer for examination. The writer would 

 therefore express his sincere thanks to Mr. Logan for his courtesy. 



Sheffield Scientific School, 

 Yale University 



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