BRYOZOA. 997 
Atactoporella ramosa, ] 
a little larger, angular, with very thin walls, and while the mesopores are readily 
distinguishable and one or two to each zocecium, the acanthopores are so small as 
to be practically wanting. 
Internal characters: To obtain the fully matured characters of this species it 
is necessary to make the tangential section as nearly superficial as possible. In 
this outermost region the zoccial wells are of moderate thickness and inflected more 
or less strongly at the points occupied by the acanthopores. The mesopores here 
appear as mere irregular interstices between the rounded walls of the zocwcia. At 
deeper levels in the section the acanthopores become smaller and at last indetermin- 
able, the zocecia prismatic and their walls thin, and the mesopores more distinct. 
The appearances now are just as in transverse sections of the axial region. Vertical 
sections show that cystiphragms and diaphragms are developed throughout the 
tubes from their origin in the axial region to their superficial orifices. These struc- 
tures are, however, a little more closely arranged near the surface of the zoarium 
than in the axial region. Mesopores are likewise present in both regions, but these 
seem gradually to expand as they bend out of the axial region and to assume the 
characters of true zocecia. At the same time new mesopores are interpolated, but 
these do not develop into zowcia. On the contrary the tendency is to close them up 
by deposit on the zocecial walls. 
This is the second species of the genus in which the zoarium rises into the 
ramose form. The first is the A. newportensis Ulrich, from the lower beds of the 
Cincinnati group at Newport, Ky. The latter has larger monticules, more numerous 
mesopores, less abudant acanthopores, and thinner zocecial walls. The ramose habit 
of growth will distinguish it from the other species of the genus. Several species 
externally similar, but internally widely different, occur in the same beds with 
A. ramosa, One of these is the Callopora pulchella, var. persimilis, another is the 
Homotrypa tuberculata of the present work, while a third is the Batostoma montuosum. 
With a little practice these will be distinguished readily enough, even without the 
aid of thin sections. When the surface is a little worn the student may find it diffi- 
cult to separate the species from montiferous examples of Monticulipora arborea, a 
species found chiefly in the overlying Galena shales. When both are in a good 
state of preservation he will find that the walls of the Monticulipora are somewhat 
thicker and much more minutely granulose, and that true mesopores are wanting. 
Formation and locality.—Upper third of the Trenton shales, near Cannon Falls, Minnesota, where it 
is associated with an abundance of Prasopora conoidea and Phylloporina corticosa. 
