MARYLAND GEOLOGICAL SURVEY 755 



tributed, small papillae ; length of cross-section about one-sixth the diam- 

 eter; width about one-twentieth the diameter. 



The species differs from other species of Micrabacia from the Coastal 

 Plain in the greater sharpness and smoothness of the basal costae, the 

 greater irregularity in the distance of the bifurcations of the several cycles 

 from the center, the greater length of the costa? of the last cycle, the greater 

 number of intercostal perforations, and the greater size attained by the 

 adults. It is distinguishable from M. rotatilis var. georgiana 1 by its 

 smoother and slightly thicker costas. M. americana Meek and Hayden, 

 and its variety, multicostata, 1 have more strongly denticulate bases. In 

 M. coronula (Goldfuss) of the European Cretaceous the denticulations of 

 the septa! edges are markedly coarser than those of any of the American 

 species. 



Dimensions (of the type). Diameter 9 mm., height about 4 mm. 



Occurrence. MONMOUTH FORMATION (Exogyra costata zone). Bed 

 of small branch about seven-eighths of a mile southwest of Brightseat 

 and three-eighths of a mile south of the Sheriff road; near McNeys 

 Corners, about a mile west of Friendly ; questionably near Seat Pleasant, 

 Prince George's County. 



Collection. Maryland Geological Survey, on deposit in the U. S. 

 National Museum. 



MICRABACIA MARYLANDICA sp. nov. 

 Plate XLVIII, Figs. 1-4 



Description. Corallum low to moderately high, subdiscoidal ; base flat 

 or slightly convex ; top evenly convex with a small axial depression about 

 1.25 mm. deep in the type. 



The underside of the base or wall is ornamented with a system of 

 radiating bifurcating cost<e which alternate with the septa; the system 

 starts with six costse which, by successive bifurcations, form cycles of 

 12, 24, 48, and 96 costas. Each of the original six costffi (first cycle) is the 

 focus of a group ; the original of each group splits near the center into two 



1 Described by the writer in Prof. Paper U. S. Geol. Survey, No. 98J, now in 

 press. 



