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ARGA (STEITOAHGA) DEE^^TSHA n. sp. 



Descripti on .- "^hell small, well inflated, decidr-:dlj inequi- 

 lateral; almost vertically subtr-onoated anteriorly, produced and 

 abruT^tlj roundeJ. posteriorly, "base gently arcuate; ujnbones full, 

 broad, relatively prominent, strongly prosofyrate, placed almost 

 at the anterior fifth of the length; posterior ridge broad and low; 

 external sculpture of numerous, narrow, wealvly sculptured radial 

 ribs of varying width, being widest posteriorly; concentric sculp- 

 ture of irregularly spaced incremental lines and striae, slightly 

 affecting the rc.dial ribs; cardinal area entirely opisthodetic, 

 narrow, impressed, crosser" by three oblioue, nlightly raised lines; 

 anterior series of teeth consisting of siz thickened, irregular 

 teeth placed under the umbo, the thickened area ceasing abruptly 

 behind the umbo and succeeded immedia'^ely by the pos-^erior series, 

 the latter numbering twenty-three teeth ^^vhi'^h increase in size to- 

 ward the distal end an? at "frhe en-'', of the series becone oblioue 

 in position; inner mar-in of the valve smooth. 



Dimensions .- Length, 16 mm.; alt., 10.5 mm.; semidiam., 

 4.1 mm.; length of hinge, 9 mm. 



Remarks .- Only a single valve of this interesting form is 

 present. The shape and soulcture suggest Oalloarca; the hinge 

 Argina; but the cardinal area is unique. 



Occurrence.- lov/er Miocene: Bor/den beds, Bowden, Jamaica. 



