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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Fig. 4. — Atribonium simatum n. sp., Newton Creek Limestone, Mich., USNM 

 141973. Longitudinal section through midline, X5, showing profiles of camaro- 

 phorium, hinge plate, and septum of spondylium. 



Comparisons. — Atribonium simatum is characterized by its sub- 

 trigonal outline, rather flat valves with most of thickness of shell pro- 

 vided by the rather abrupt geniculation of the valves near the anterior, 

 its costate fold and flanks, with normally four but as many as seven on 

 the fold, its flat or slightly emarginate anterior, and sharp beak with 

 distinct beak ridges. It resembles A. cooperorum n. sp. from the Bell 



Fig. 5. — Atribonium simatum n. sp., Newton Creek Limestone, Mich., trans- 

 verse sections X6, USNM 141977, original length: ca. 11.5 mm., all measure- 

 ments from pedicle valve beak. 



A. 0.6 mm. : camarophorium just a swelling on septum, intercamarophorial 

 plate nearly duplex. B. 0.7 mm. : camarophorium wider. C. 0.8 mm. : cardinal 

 process higher. D. 1.0 mm. : maximum height of cardinal process. E. 1.2 mm. : 

 hinge plate detached from walls. F. 1.5 mm. : intercamarophorial plate thin, 

 crural bases apparent. G. 1.6 mm. : intercamarophorial plate gone. H. 1.9 mm. : 

 crura present. I. 2.5 mm. : no crura. 



