MARYLAND GEOLOGICAL SURVEY 21? 



polygonal apertures, 10 or 11 of the average size in 3 mm. Clusters of 

 zocecia, some of them attaining a diameter of 0.37 mm., occur at intervals 

 of less than 4 mm. Mesopores comparatively few, often difficult to detect 

 at the surface. 



" Internal Structure : Tangential sections show that the zocecial walls 

 are polygonal and very thin, with neighboring cells in contact, except at 

 many of the angles of junction, these being occupied by one or two small 

 mesopores. The latter often form very inconspicuous clusters at the 

 center of the groups of large zooacia, but in the intermediate spaces not 

 over half of the angles of junction between the ordinary zocecia are 

 occupied by mesopores. A few very small acanthopores are developed. 

 The opening left by the cystiphragms is generally of ovate form and more 

 often eccentric than central in its position within the tube cavity. 



" Vertical sections are peculiar chiefly because they exhibit a marked 

 decrease in the number of mesopores when compared with other species 

 of the genus." Ulrich, 1893. 



Occurrence. CHAM:BERSBURG LIMESTONE (Nidulites bed). Pennsyl- 

 vania, Maryland, and Virginia. The cliffs along Conococheague Creek 

 at Wilson, Maryland, exhibit specimens of this bryozoan. A not uncom- 

 mon fossil of the Decorah shale division of the Black Eiver group of 

 Minnesota. 



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



Family CONSTELLARIIDAE . 



Genus DIANULITES Eichwald 

 DlANULITES PETROPOLITANUS Dybowski 



Plate XLIV, Figs. 6, 7 



Dianulites petropolitanus Dybowski, 1877, Die Chaetetiden der Ostbaltischen 



Silurform., p. 24, pi. 1, figs. 4, 5. 

 Diamilites petropolitana Bassler, 1911, Bull. U. S. Nat. Mus. No. 77, pp. 232, 



237, pi. ii, figs. 4-6a; pi. x, figs. 7-11; text figs. 129-132. 

 Monotrypa (Chaetetes ?) cumulata Ulrich, 1893, Geol. Minnesota, vol. iii, 



pt. 1, p. 307, pi. xxvii, figs. 26, 27. 



Description. Zoarium massive, usually hemispheric with a slightly 

 concave, epithecated base, and about 2.5 cm. wide, but ranging from this 



