226 THE PALEONTOLOGY OF MINNESOTA. 



lAtactoporella ramosa. 



is shown in figures 18 to 20 (pi. XV), consist of a gradual thickening of the zooecial 

 walls, and strengthening of the acanthopores, causing the mesopores. which in the 

 thin-walled regions are large and distinct enough, though never as numerous as in 

 most other species of the genus, to be more or less completely obliterated. Cysti- 

 phragms occur in every zooecial tube. The opening in them varies from subcircular to 

 semiovate, according as they are situated subcentrally or at one side of the zocecium. 

 Vertical sections (pi. XV, fig. 21) show that the cystiphragms and diaphragms in both 

 sets of tubes are equally crowded. It is, therefore, difficult to distinguish the meso- 

 pores from those zooecial tubes in which the curved section of the cystiphragms is 

 not shown. 



This is a well marked species and readily separated from A. schucherti Ulrich, 

 and certain varieties of A. insueta, which it resembles chiefly in the large size of the 

 acanthopores, by the much greater abundance of transverse partitions in the tubes. 

 The zoarium is also heavier, and the zoo3cial walls thicker. A nearer relative 

 apparently than either of the species compared, is found in an undescribed species 

 occurring near the tops of the hills at Cincinnati, Ohio. 



Among associated species that might be confounded there are two of Leptotrypa, 

 one, a parasitic undescribed form, the other, subglobular, described in this volume 

 as L. acervulosa. Both are distinguished externally by having much thinner walls 

 and inconspicuous acanthopores. When sectioned the Leptotrypas will of course be 

 separated at once by the total absence of cystiphragms. 



Formation and locality. Galena shales, St. Paul and near Cannon Falls, Minnesota. 



ATACTOPORELLA RAMOSA, n. sp. 



PLATE XX. FIGS. 22-27. 



Zoarium erect, subramose, branches compressed, dividing once or twice, 3 to 6 

 mm. thick, 5 to 10 mm. wide. Surface minutely spinulose, and elevated at intervals 

 of about 2 mm. into small monticules, varying in different examples from low and 

 rounded to prominently conical in shape. Slopes of monticules occupied by cells of 

 slightly larger size than the average ; their summits, however, often appear solid, 

 these being, in most cases, formed by small aggregations of closed mesopores. 

 Zoojcial apertures small, floriform, about sixteen in 3 mm., separated by interspaces, 

 in which but few mesopores can be detected, of less width than their diameter; 

 each surrounded by from five to seven small acanthopores, projecting well into the 

 zoo3cial cavity. This describes the fully matured and ordinary appearance of well 

 preserved specimens. Young stages are quite different, the zooacial apertures being 



