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siilDiinbricated sciilptiire. Typically the posterior margin is eons-oi- , 

 cuousl;- proauced "belov^. 



Remarlis .- A. "b^v^deniana is very closely relatel to the assoc- 

 ciated A, nmhonata . There appeare, however, to be constant diff- 

 erences between them. A, bowdenigna is smaller and usually less 

 inflater\ has slightly more -orosos-yrate beaks, finer sculpturing, 

 lari?er teeth, a somewhat narrower cardinal area, a stronger post- 

 erior keel- vihioh forms at the lower end of the posterior margin 

 a usually prominent "^Ing"- and the region dorsal to the posterior 

 keel is more excavated. But perhaps the most notable distinction 

 lies in the differences in diameter of the various regions of the 

 shell. In the case of bov/deniana the maximum diameter is at about 

 the middle, i. e., posterior to the beaks, the diameter rapidly 

 decreasing anteriorly acrocs the beaks to the anterior margin; while 

 umbonata h as the greatest diameter at, or just posterior to, the 

 beaks and from that point the decrease anteriorly to the margin 

 is even more rapid. 



A. granimatodonta Dall (l) , of the Tampa silex ber!s fauna, 

 is much larger, lees inflate'!, the slope from the umbo to the 

 anterior margin is less steep and the posterior slope is not as 



(1). Dall, '.7. H., Bull. U. S. Fat. Mus., no. 90, v. 118, pi. 

 20. fiffS. 1, 2, pi. rs, fi!?. 3, 1915. 



