CYRTINA 289 



valve is marked by three or four rounded plications which originate at 

 the cardinal margin, the two plications bounding the sinus are much the 

 stronger, the outermost ones becoming much less distinct and sometimes 

 nearly obsolete. Internally a strong median septum reaches nearly or 

 quite half way from the beak to the front margin. 



Brachial valve depressed convex, sometimes nearly flat, compressed 

 towards the cardinal extremities ; mesial fold rounded, originating at the 

 beak, scarcely elevated above the plications on either side; each lateral 

 slope marked by two or three plications, the outermost ones being more or 

 less indistinct. Internal characters not observed. 



Besides the plications the surface of both valves is marked by concen- 

 tric lines of growth which are frequently rather strong and somewhat 

 crowded towards the front margin. Shell structure minutely punctate. 



Remarks. The shell which has been identified as C. burlingtonensis oc- 

 curs most commonly in the Chouteau limestone fauna, It differs from 

 C. acutirostris of the Louisiana limestone in its narrower and more elongate 

 form with more attenuate beak, in the rounded lateral margins of the 

 cardinal area, and in the comparatively narrower plications next to the 

 mesial fold of the brachial valve. The species was originally described 

 by Rowley from the white cherts at the base of the Burlington limestone 

 at Louisiana, Missouri, and it has been recognized in the Fern Glen fauna. 



Horizon. Chouteau limestone, Fern Glen formation and base of the 

 Burlington limestone. 



CYRTINA NEOGENES Hall and Clarke 

 Plate XXXV, Figs. 32-38 



1895. Cyrtina sp. '? Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 39, 



figs. 25-28. 

 1895. Cyrtina neogenes Hall and Clarke, Pal. N. Y., vol. 8, pt. 2. pi. S4, 



fig- 41. 

 1897. Cyrtina neogenes Hall, 14th Rep. N. Y. State Geol., p. 372, pi. 8, 



figs. 4-8. 



Description. Shell below medium size or small, broader than long, the 

 hinge-line shorter than the greatest width of the shell, the cardinal ex- 

 tremities rounded. The dimensions of two specimens are: length of 

 pedicle valve 15 mm. and 7.1 mm., length of brachial valve 11 mm. and 

 6.5 mm., width 16.8 mm. and 11 mm., thickness 14 mm. and 6 mm., length 

 of hinge-line 13 mm. and 8.7 mm., height of cardinal area 5.5 mm. and 

 2.3 mm. 



Pedicle valve strongly convex, the convexity extending out to the car- 

 dinal extremities; the mesial sinus sharply defined, non-plicate, rather 

 deep and of moderate width, subangular in the bottom; cardinal area 

 rather high, concave, becoming more strongly arched towards the beak, 

 10 



