314 THE PALEONTOLOGY O^ MINNESOTA. 



[Nieholsonella pulchra 



NiCHOLSONELLA PULCHRA, «. Sp. 

 PLATE XXI. FIGS. 8-12. 



Zoarium forming a bushy mass, consisting of irregularly divided, sometimes 

 anastomosing flattened branches, 6 to 10 mm. thick and 8 to 20 mm. wide. Surface 

 with small conical or rounded menticules, subsolid at their apices, and frequently 

 uniting on the rounded edges of the branches to form short ridges. In some speci- 

 mens the monticules are very slightly developed. Zooecial apertures rounded, small, 

 subequal, regularly arranged, about eleven in 3 mm., separated by interspaces nearly 

 equalling their diameter — about 0.15 mm. Interspaces minutely papillose, generally 

 depressed midway so that a rather irregular ring of papillae surrounds each aperture. 

 Mesopores, though completely isolating the zooecia, are to be detected at the surface 

 only in young and weathered examples. 



Internal characters: In vertical sections the tubes diverge with comparative 

 rapidity and uniformity of curvature. Their walls are thin, though not excessively 

 so, and exhibit that lack of sharpness which characterizes especially the Trenton 

 species of the genus. Young zooecial tubes arise in the axial region mainly and 

 expand very gradually. Diaphragms occur throughout, two or three times their 

 diameter apart in the axial region, and averaging nearly twice as many in a given 

 space in the peripheral region. In young examples it is not easy to distinguish the 

 mesopores from the true zooecial tubes, but the solid deposit which more or less com- 

 pletely fills up the outer part of the mesopores in the fully matured stages then 

 renders the task an easy one. This deposit is lined vertically with rows of dots, and 

 in many cases is divided up iuto two or more layers with light intervals between them. 

 The two halves of fig. 10 show, in the upper, the structure of a matured example just 

 beneath the surface. Here the zooecia are as usual not sharply defined and the inter- 

 spaces completely filled with solid tissue in which a great number of small dots 

 (representing the superficial papillae) are to be observed. At a deeper level in the 

 zoarium (see lower half of fig. 10) the large angular mesopores are open. Here even 

 some dots (?acanthopores) are to be made out in the walls, chiefly at the angles of 

 junction. 



Transverse sections show that in the axial region the tubes are of all sizes and 

 variously angular. No dots like those seen in tangential sections are to be detected, 

 but one of my sections exhibits fairly conclusive evidence of an intermittent struc- 

 ture of the walls not unlike fig. 26 on plate 27. 



