RHYNCHOPORA 233 



versely from the median line the surface is at first gently convex and 

 then curves abruptly to the lateral margins, the curvature being a little 

 broken towards the front by the differentiation of the median fold, the 

 lateral slopes are strongly arched from beak to front with a more abrupt 

 curvature to the front; the mesial fold is obsolete in the posterior half 

 of the valve and is but moderately elevated above the lateral slopes in. 

 front ; the beak incurved beneath that of the opposite valve. Internally 

 the cardinal process is absent and the hinge-plate is undivided; a strong 

 median septum is present which is divided posteriorly, each branch sup- 

 porting one side of the hinge-plate, between the divisions of the septum 

 is a crural cavity triangular in cross section, covered on one side by 

 the hinge-plate and open only towards the front, anteriorly the septum 

 is free and reaches from one-third to one-half the length of the valve. 



The surface markings of the shell, aside from the plications, are not 

 clear on the internal casts, but fragments of external moulds indicate 

 that it was nearly or quite smooth. The shell structure is finely and 

 closely punctate, the punctas being much more abundant upon, and seem 

 sometimes to be confined to, the tops of the plications. The punctas render 

 the surface of the internal casts finely papillose, and when the internal 

 cast and mould of the exterior are examined together the two surfaces 

 are joined by innumerable, minute, transverse pillars which are the 

 siliceous casts of the perforations through the shell. 



Remarks. This species resembles R. pustulosa but has somewhat finer 

 and more numerous plications, and the entire aspect of the shell is more 

 angular in form. The internal characters of the species are identical in 

 all fundamental respects, but the dental lamellae of the pedicle valve 

 are somewhat longer, and the divided portion of the septum of the 

 brachial valve seems to continue further towards the front in R. beecheri. 

 The species is most commonly preserved as internal casts and external 

 moulds in the cherts of the Keokuk limestone, and it has not been observed 

 in any other condition by the writer. The punctate character of the 

 shell and the internal rostral characters are easily recognized in the chert 

 specimens. 



Horizon. Residual Keokuk chert. 



RHYNCHOPORA PERSINUATA (Winchell) 

 Plate XXX, Figs. 1-6 



1865. Rhynchonella persinuata "Winchell, Proc. Acad. Nat. Sci. Phil., 



p. 121. 

 1883 Camarophoria occidentalis Miller, Jour. Cinn. Soc. Nat. Hist., vol. 



4, p. 313, pi. 7, figs. 7-7b. 

 1901. Camarotoechia persinuata Weller, Trans. St. Louis Acad. Sci., vol. 



11, p. 197, pi. 19, fig. 5. 



