314 THE PALEONTOLOGY OF MINNESOTA. 
[Nicholsonella pulchra,. 
NICHOLSONELLA PULCHRA, 2. 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. Zocecial 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 papille surrounds each aperture. 
Mesopores, though completely isolating the zocecia, 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 zocecial 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 zocecial 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 into 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 zocecia 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 papillz) are to be observed. At a deeper level in the 
zoarium (see lower half of fig, 10) the large angular mesopores aré 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. 
