RHYNCHOTETRA 211 



once near or in front of the middle, making twelve to fifteen plications 

 in all upon the lateral and anterior margins ; internally the dental lamel- 

 lae are strongly developed and are joined to form a spondylium supported 

 by a median septum, as has been shown in the generic description. 



Brachial valve more convex and deeper than the pedicle, the greatest 

 depth near the middle, the surface curving from the central point with 

 a regular convexity in all directions; along the postero-lateral margins 

 the surface is abruptly deflected towards the opposite valve, the deflected 

 portion forming the larger part of the flattened or concave areas on each 

 side of the beak ; mesial fold obsolete ; the beak small, pointed, incurved 

 beneath that of the opposite valve ; the plications entirely similar to those 

 of the opposite valve, but with the later ones added by intercalation 

 rather than by division. Internally, a median septum is present which 

 is divided internally to form a crural cavity, as has been shown in the 

 generic description. 



The minute surface markings consist of fine, radiating striae. Lines 

 of growth of varying strength may be present and are sometimes a little 

 crowded towards the anterior margin. 



Remarks. This species most closely resembles E. gibbosum, but it is a 

 much thinner species, with much less convex valves, and with the great- 

 est width more anterior in position ; in the form and arrangement of the 

 plications, however, the two species are much alike. 



Horizon. Burlington limestone. 



RHYNCHOTETRA GIBBOSUM (Greger) 

 Plate XXVII, Figs. 7-15 



1894. Rhynchonella sp. (?) Keyes, Mo. Geol. Surv., vol. 5, pi. 41, fig. 8. 

 1910. Paraphorhynchus gibbosum Greger, Am. Jour. Sci. (4), vol. 29, 

 p. 73, figs. 1-6. 



Description. Shell strongly plicate, of medium size or larger, longer 

 than wide, the greatest width near or in front of the middle, the anterior 

 outline usually subsemicircular, the posterior outline acutely subcuneate. 

 The dimensions of a nearly complete specimen, a cotype, are: length 

 39 mm., width 25.5 mm., thickness 28 mm. 



Pedicle valve arched from the beak to the front, but somewhat more 

 flattened posteriorly, arched to the antero-lateral margins, the postero- 

 lateral margins abruptly deflected towards the opposite valve, the de- 

 flected portion forming nearly one-half of an oval, flattened or concave 

 lateral region upon each side of the shell; mesial sinus obsolete in the 

 posterior portion of the shell, broad, shallow, obscure and ill-defined 

 anteriorly; beak small, pointed, a little incurved; plications strong, 

 rounded on top, separated by broad, rounded furrows, from three to five 

 originate near the beak, all or nearly all of which divide at intervals 



