220 MISSISSIPPIAN BRACHIOPODA 



margin and is often more or less abruptly thickened near its anterior ex- 

 tremity, this septum supports a spondyliuin whose spreading sides meet 

 at an angle in the bottom. 



Braehial valve much deeper than the pedicle, the greatest depth near 

 the front margin; from this point the surface slopes with a 

 gentle convex curvature to the beak, and is abruptly deflected to 

 the anterior and antero-lateral margins, along the postero-lateral margins 

 the surface is abruptly bent towards the opposite valve and is usually 

 somewhat inflected and forms more than one-half of the smooth, oval, 

 concave areas on each side of the beak ; mesial fold obsolete posteriorly, 

 broad and ill-defined anteriorly; the beak is strongly incurved beneath 

 that of the opposite valve and partly fills the delthyrium of that valve, 

 the plications are similar to those of the opposite valve and are alternate 

 with them, as are the serrations on the margin, the plications towards the 

 postero-lateral margins are strongly arched. Internally the valve bears 

 a strong median septum which extends from the beak more than half way 

 to the line of deflection of the valve towards its anterior margin, in its 

 anterior half this median septum bisects a subovate or subelliptical mus- 

 cular impression, at the beak of the valve it supports a subquadrangular, 

 spoon-shaped hinge-plate whose ventral surface is depressed medially 

 through about one-third its width, the lateral margins of this depressed 

 region diverge from a point at the beak and are overhanging so as to give 

 to the depressed region the appearance of a flattened, conical tube cleft 

 along one side. 



In addition to the plications each valve is marked only by concentric 

 lines of growth which are obscure or obsolete over the greater portion of 

 the shell, ordinarily being clearly recognizable only upon the deflected 

 portion of the valves near the anterior margin. 



Remarks. This species has been most commonly recognized in the re- 

 sidual cherts of the Keokuk limestone, where it occurs in the condition of 

 internal casts showing clearly the median septum in each valve and the 

 muscular impressions of the brachial valve. Where it has been found 

 in situ it is known only in the Keokuk limestone. Hall and Clarke 1 have 

 illustrated an internal cast under the name Camarophoria ringens Swallow, 

 which is clearly an example of Meek and Worthen's Rhynchonella sub- 

 trigona, and more recently a specimen of the same species has been figured 

 by Greger 2 as typical of Swallow's species. Swallow's type has been de- 

 stroyed but the specimen illustrated by Greger is said to bear a label writ- 

 ten by Swallow himself and is therefore considered by that writer to be 

 authentic, a conclusion which, if true, would necessitate Swallow's name 

 taking precedence over that of Meek and Worthen. A careful examina- 



1 Pal. N. Y., vol. 8, pt. 2, pi. 84, fig. 45. (1895.) 



2 Am. Jour. Sci. (4), vol. 29, p. 71. figs. 7-8. (1910.) 



