BKADLEY: BIL^CHIOPODA of the MAQUOKETA of IOWA. 513 



Dinorthis proavita at Clermont, pro))ably the most common species, 

 is smaller than most of the other Orthidae. 



Locality: — Common in the upper portion of the ]Maquoketa at 

 Spring Valley, IMinnesota and ^Yihnington, Illinois; Lower ]Maquo- 

 keta at Clermont, Iowa. ]M. C. Z. S,o3S. 



Strophomexidae. 



PLECTAikiBOXiTEs RUGOSus (Meek). 



Leptaena rugosa James, Cat. fossils Cincinnati group, 1871. 



LejAaena sericea var. rugosa ]Meek, Pal. Oliio, 1873, 1, pt. 2, pi. 5, fig. 3f, g, h. 



Plectambonites rugosa Foerste, Bull. sci. lab. Denison univ., 1912, 17, p. 123, 



pi. 1, fig. 7a-e; pi. 10, fig. 7a-d. 

 Leptaena aspera James, Cine, quart, journ. sci., 1874, 1, p. 151. 

 "? Plectambonites sericeus var. Ruedemann, Bull. 49, X. Y. state mus., 1901, 



p. 18, pi. 1, fig. 6, 7; ibid., 8, p. 525. 



The typical specimens of this species were found in the Eden at 

 Cincinnati, Ohio. The name rugosa was applied because of the 

 roughened exterior surface and the oblique -OTinkles along the hinge- 

 line. 



The concentric banding and filming is often absent as well as the 

 conseciuent thickening of the \'alves anteriorly. This is the case with 

 most of the specimens from the ^Slaquoketa near Clermont, Iowa; 

 the surface is comparatively smooth except for concentric striae 

 present on some forms. 



Plectambonites rvgosus has a considerable vertical range; it is most 

 abundant in the Eden, but similar forms have been found in the 

 Waynesville and Liberty members of the Richmond. ]Much varia- 

 tion of outline occurs within the species, and on the same slab examples 

 may be seen representing all stages between the semielliptical and the 

 elongate quadrangular. These shells grow more in width than 

 length, and environment often differentiates them so greatly as to 



