92 MISSISSIPPIAN BBACHIOPODA 



cardinal area narrow, the cardinal process small. Internally the mus- 

 cular scars are of only moderate strength, and the surface papillae are 

 well developed. 



Surface of both valves marked by very fine, subangular costse, which 

 increase by intercalation and by division, about eight or more occupying 

 the space of 1 millimeter, from 75 to 100 being present upon each valve 

 depending upon the size of the valve. Crossing the costae are minute, 

 concentric, raised lines, which are undeveloped in the intercostal furrows, 

 and which give to the costae when the surface is perfectly preserved, a 

 crenulate appearance. 



Remarks. In its size and general form this species resembles C. 

 geniculatus, but it differs conspicuously from that species in its much 

 finer costae, there being twice, or more than twice, as many in this 

 species upon shells of equal size. The concentric markings of the shell 

 are also different, they being of the same type as those of C. logani 

 and C. ornatus; the pedicle valve is less convex and the brachial valve 

 less concave than in C. geniculatus. The more minute surface markings 

 of this species are commonly obliterated in the usual condition of preser- 

 vation of the shells, and only one individual observed among a great 

 number has satisfactorily shown the concentric markings which appear 

 as crenulations of the costas. In no case have the cardinal spines of 

 the species been observed and upon the great majority of specimens 

 even the spine bases cannot be detected. 



Horizon. Kinderhook. 



CHONETES GENICULATUS White 

 Plate VIII, Figs. 35-42 



1862. Chonetes geniculata White, Proc. Boston Soc. Nat. Hist., vol. 9, p. 29. 

 1894. Chonetes geniculatus Keyes, Mo. Geol. Surv., vol. 5, p. 53, pi. 38, 



fig. 3. 

 1908. Chonetes geniculatus Rowley, Mo. (Bureau Geol. and Mines, vol. 8, 



2nd ser., p. 75, pi. 17, figs. 15-19. 



Description. Shell small, subsemielliptical in outline, rather strongly 

 concavo-convex, the convexity extending well out towards the cardinal 

 extremities, the length about five-sixths of the width, the length of the 

 hinge-line equal to, or a little shorter than, the greatest width of the 

 shell, the cardinal angles rectangular or a little obtuse, lateral margins 

 nearly straight or slightly convex posteriorly, rounding into the anterior 

 margin in front. The dimensions of two specimens are: length 5 mm. 

 and 5.7 mm., width 6 mm. and 6.9 mm., convexity 1.8 mm. and 2 mm. 



Pedicle valve most convex near the middle, a little compressed towards 

 the cardinal extremities, but the auriculations are small and not sharply 

 differentiated from the body of the shell; mesial sinus obsolete, but 



