MARYLAND GEOLOGICAL SURVEY 215 



diameter of 0.25 mm. Clusters of larger zooecia form the monticules, or 

 are scattered over the surface of the smooth zooecia at intervals of about 

 2.5 mm., measuring from center to center. These zocecia measure from 

 0.30 to 0.38 mm. in diameter. At the center of the monticules a few 

 mesopores, or more probably young zooecia, are present. Acanthopores 

 rather large and numerous. 



In longitudinal sections the zocecial tubes exhibit thin walls with the 

 characteristic granulose structure. The diaphragms are curved and occur 

 at intervals of a tube-diameter or more in the lower half of the tubes and 

 about one-third that distance apart near the surface. In tangential sec- 

 tions the zocecial walls are angular and thin with a strong acanthopore at 

 most of the angles. The method of growth, small, angular, thin-walled 

 zocecia, large acanthopores, granulose wall structure and the presence of 

 curved diaphragms are the important features of this characteristic and 

 wide-spread Lowville fossil. 



Occurrence. CHAMBERSBTJRG LIMESTONE (Tetradium cellulosum 

 bed). Several localities south of Chambersburg in Franklin County, 

 Pennsylvania. Lowville limestone of the Ohio Valley, etc. 



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



Genus PRASOPORA Nicholson and Etheridge, Jr 

 PRASOPORA INSULARIS Ulrich 



Plate XLIV, Figs. 15-17 



Prasopora insularis Ulrich, 1893, Geol. Minnesota, vol. iii, p. 251, pi. 16, 

 figs. 18-23. 



Description. " Zoarium small, discoid, piano- or concavo-convex, com- 

 monly from 15-20 mm. in diameter and 5 or 6 mm. in height. In a very 

 large example these dimensions are respectively 28 and 12 mm., while in 

 the smallest seen they are 1.5 and 0.5 mm. Under surface with a central 

 scar, and beyond it delicate radiating lines, fine concentric striae, and, at 

 intervals indicating stages of growth, stronger wrinkles. Very often the 

 zoaria are evidently made up of distinct superimposed layers, but these 

 are not usually distinguishable internally. Upper or convex surface with- 

 out monticules, but exhibiting, at intervals of about 4 mm., distinct 



