224 SYSTEMATIC PALEONTOLOGY 



enclose the elliptical primitive aperture. In the succeeding stages the 

 principal change is a reduction in the size of the apertures, caused by an 

 internal deposit." Ulrich, 1893. 



Occurrence. CHAMBERSBURG LIMESTONE (Caryocystites bed). Blue 

 Spring, Franklin County, Pennsylvania. The type specimens of this 

 species were found in the Decorah shales division of the Black Eiver at 

 Minneapolis, Minnesota. 



Collections. Maryland Geological Survey, U. S. National Museum. 



Genus ARTHROPORA Ulrich 



AETHEOPORA BIFURCATA Ulrich 



Plate XLVIII, Figs. 13-15 



Arthropora bifurcata Ulrich, 1893, Geol. Minnesota, vol. iii, p. 178, pi. xiv, 

 figs. 22-25. 



Description. " Segments small, thin, with sharp edges and rather wide 

 nonporiferous border, the lower ones bifurcating, usually only once; so 

 far as observed not over 8 mm. long, and from 1.2 to 1.8 mm. wide; the 

 upper joints shorter, their length occasionally less than 5 mm., bifurcat- 

 ing or with a single lobe-like projection on one or both sides. Young 

 segments with comparatively large, ovate zocecial apertures, not very 

 regularly arranged in longitudinal and diagonally intersecting series, 

 with about nine in 3 mm. lengthwise, and five in 1 mm. diagonally. 

 Apertures enclosed in distinct granulose rims, connecting longitudinally. 

 Interspaces depressed, sometimes with a few indistinct striae. With age 

 the zocecial apertures become more circular and smaller, and the peri 

 stomes and connecting ridges thicker." Ulrich, 1893. 



Occurrence. CHAMBERSBURG LIMESTONE (Christiania bed). Green- 

 castle and other localities in southern Pennsylvania. 



Black Eiver and early Trenton of Minnesota, Kentucky, Tennessee, 

 and Canada. 



Collections. Maryland Geological Survey, U. S. National Museum. 



