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PROCEEDINGS OF THE NATIONAL MUSEUM. 



that there were a number of species and varieties grouped under A. 

 gemma. The species referred to the genus at present are: 



Species (it present referred to Acrotretn. 



ACROTRETA ARGENTA, new species. 



The material representing- this species is uiore or less crushed and 

 broken in the shaly sandstone and limestone, but a few specimens 

 show its general form and characters. It belongs to the A. iddhoemis 

 group in having a wide false area, strongly marked apical callosity, 

 and foraminal tube. The thickening in the posterior portion of the 

 brachial valve is also characteristic of the two species. 



The cast of an interior of the pedicle valve reveals what 1 have 

 searched for unsuccessfully in hundreds, if not thousands, of speci- 

 mens of this genus. 1 refer to the visceral area with the position of 

 the point of attachment of the central and lateral muscles on each side 

 of it, essentiallv as in Oholella and Obolm. 



