310 S. WELLES FAUNA OF THE FERX GLEX FORMATION 



costata by its larger size, its much larger cardinal area, and its propor- 

 tionately smaller plications. The presence of a median septum in the 

 shell has not been established. 



CTRTINA BURLIXGTOXEXSIS Rowley 

 Plate 13. figures 20-23 



1893. Cyrtina burlingtonensis Rowley. American Geologist, volume 12, page 

 308, plate 14, figures 15-17. 



Description. — Shell obliquely subpyramidal in form, the hinge line 

 usually a little shorter than the greatest width of the shell, and the car- 

 dinal extremities rounded. Pedicle valve strongly convex, the beak acu- 

 minate and incurved over the cardinal area ; cardinal area high, strongly 

 arched posteriorly, its lateral margins not sharply defined, but rounding 

 regularly into the lateral slopes of the shell; delthyrium narrow, closed 

 by a rather strongly convex pseudo-deltidium which is pierced by a small 

 foramen situated close up under the beak; surface of the valve marked 

 by from three to five rounded plications upon each lateral slope ; the two 

 median plications are the larger and extend to the beak; the remaining 

 ones grow successively smaller toward the cardinal extremities and be- 

 come obsolete near the cardinal margin; between the two median plica- 

 tions is a rounded median sinus which originates at the beak. Brachial 

 valve depressed convex, sometimes nearly flat, wider than long, marked 

 by plications similar to those of the pedicle valve; the median plication 

 is the broadest, although it is but slightly elevated above the adjacent 

 lateral ones. In addition to the plications, the surface of each valve is 

 marked by more or less irregular concentric lines of growth. 



The dimensions of two nearly perfect specimens from the Chouteau 

 limestone are: Width of shell, 13 millimeters and 9.8 millimeters; length 

 of hinge line, 11.5 millimeters and 7.5 millimeters; length of pedicle 

 valve from front to beak, 13.6 millimeters and 10 millimeters; height of 

 area from hinge line to tip of beak, 5 millimeters and 4 millimeters; 

 length of brachial valve, 9.5 millimeters and 7 millimeters. 



Remark s. — This species was first described from the white cherts at 

 Louisiana, Missouri, but it seems to occur most commonly in the Chou- 

 teau limestone of Pettis county, Missouri. In the Fern Glen fauna a 

 single specimen, a somewhat crushed and distorted pedicle valve, has 

 been observed, but it possesses all the essential specific characters of the 

 Chouteau limestone examples. The species may be distinguishd from 

 other members of the genus with which it might be confused by the 

 rounded instead of angular cardinal margins. 



