4 6 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



Pedicle valve interior with dental plates converging to form spon- 

 dylium, in configuration normal for genus; spondylium normally 

 sessile in apex of beak, remaining sessile as much as 1.5 mm. along 

 floor; anterior extent of spondylium not ascertained; median septum 

 elevating anterior part of spondylium extending along floor about half 

 length of shell, as much as 6 mm. in largest specimens. 



Brachial valve interior with elevated camarophorium ; expansion of 

 crest beginning against beak of valve, septum extending anteriorly 

 slightly beyond midlength, considerably beyond midlength of valve. 



Holotype.—USNM 141822, plate 2, fig. 3b. 



Comparisons. — Atribonium cooper or win is characterized by its flat 

 convexity, evenly curved anterior margin, rather prominent fold, 

 strong costae at anterior, and sharp pedicle beak with sharp beak 



Fig. 7. — Atribonium cooperorum n. sp., Bell Shale, Rogers City, Mich. ; trans- 

 verse sections X6, USNM 141825, original length 9.1 mm., all measurements 

 from pedicle valve beak. 



A. 0.3 mm. : spondylium sessile. B. 0.5 mm. : camarophorium and cardinal 

 process visible, spondylium still sessile. C. 0.8 mm. : spondylium elevated. D. 

 1.1 mm. : hinge plate detached from walls, intercamarophorial plate separated. 

 E. 1.4 mm. : dental plates detached. F. 1.8 mm. : hinge plate small, intercama- 

 rophorial plate absent. G. 2.1 mm. : hinge plate absent. H. 2.5 mm. : camaro- 

 phorium wide. I. 2.9 mm. : camarophorium wider. J. 4.2. mm. : only septum 

 remains of spondylium, camarophorium narrow, high ; camarophorium extends to 

 4.5 mm. from beak. 



