440 MISSISSIPPIAN BRACHIOPODA 



1894. Betzia ? osagensis Keyes, Mo. Geol. Surv., vol. 5, p. 94. 



1895. Acambona ? osagensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 



51, figs. 38, 39. 



Description. Shell of medium size, longer than wide, the hinge-line 

 very short, subovate in outline, the greatest width anterior to the middle 

 of the shell, the lateral margins diverging from the beak in nearly straight 

 lines almost to the line of greatest width of the shell, the anterior and 

 antero-lateral margins describing a nearly semicircular curve. The di- 

 mensions of the type specimen as given by Swallow are : length 30.5 mm., 

 width 23.8 mm., thickness 18.8 mm. The dimensions of an imperfect, 

 somewhat crushed example are: length of pedicle valve 23 mm., length 

 of brachial valve 20 mm., greatest width 21.5 mm. 



Brachial valve most convex posterior to the middle, the umbo prom- 

 inent and projecting notably beyond the hinge-line, the surface of the 

 valve arched from beak to front along the median line, somewhat flat- 

 tened towards the front, the curvature of the surface from the umbonal 

 region towards the postero-lateral margins is rather abrupt, towards the 

 cardinal extremities becoming inflected ; the beak is prominent and in- 

 curved, and is perforated by a large, circular foramen which encroaches 

 upon the umbonal region ; the cardinal area is small and arched, its lateral 

 margins are defined and slope from the sides of the beak to the cardinal 

 extremities. 



Brachial valve less convex than the pedicle, the greatest convexity pos- 

 terior to the middle, the beak somewhat projecting beyond the hinge-line, 

 the surface curving abruptly from the umbonal region to the cardinal ex- 

 tremities and to the postero-lateral margins, the curvature more gentle 

 anteriorly and antero-laterally. 



Each valve marked by about 50 fine, simple, flattened, radiating plica- 

 tions separated by narrow grooves. The shell structure minutely and 

 finely punctate. 



Remarks. This is an exceedingly rare shell in the Chouteau limestone, 

 and only a single, imperfect, and somewhat crushed specimen has come 

 under the observation of the writer. It is essentially impossible to point 

 out any characters by which the species can be separated from 

 E. verneuiliana or E. vera. The number of plications being about 50 is 

 perhaps more nearly like E. verneuiliana. but the apparent shape of the 

 shell, with its greatest width in front of the middle, is more like E. vera. 

 Upon the specimen which has been studied the plications of the shell are 

 apparently more sharply marked upon the exfoliated portions of the sur- 

 face, and upon one small area, which is apparently in its natural condition, 

 no plications are discernable. It is possible that this may prove to be an 

 essential feature of the species, but to determine this the examination of 

 more material is desirable. 



