178 
Kirtley F, Mather 
edly different outline of the shell, are noteworthy. R, magni- 
costa is, perhaps, more closely related to such Mississippian shells 
as R. ? cooperensis (Shumard) than to either of the two Penn- 
sylvanian species which have been previously described. 
Horizon and locality. Brentwood limestone: near Fayette- 
ville, Arkansas (Stations 135 and 150). 
TEREBRATULID^ 
Genus DIELASMA King 
Dielasma subspatulatum Weller 
Plate XI, figures 1-1 lb; text figure U. 
1914. Dielasma subspatulatum. Weller, 111. Geo. Surv., Mon. I, p, 270, 
pi. 33, figs. 6-11. 
Washington County, Arkansas. 
1914. Dielasma bovidens. Kozlowski, Annales de Paleontologie, T. 9, p. 
8’8, pi. 9, figs. 61-65. 
Upper carboniferous beds: Bolivia. 
O Q ^ O 
W ViV 
Fig. 4. A series of eleven cross sections of the rostral portion of Dielasma 
subspatutulatum (x 2J), only the first four of which show the pedicle valve. 
At certain of the Arkansas localities this is one of the most 
abundant species and in all horizons of the Morrow group it is 
a common fossil. The material at hand includes specimens in all 
stages of growth from immature forms scarcely 4 mm. in length 
to individuals in old age which have a length of over 30 mm. 
All have the typical spatulate outline but considerable variation 
in the degree of flattening of the anterior portion of the pedicle 
valve is displayed. In a few individuals a shallow undefined 
