- BRYOZOA. 245 
Prasopora simulatrix.] 
PRASOPORA SIMULATRIX Ulyich. 
PLATE XVI, FIGS. 1-10. 
Prasopora simulatrix ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p, 85. 
Zoarium discoid in the younger stages, becoming hemispheric or subconical with 
age; occasionally the central part of the upper surface is drawn out, and in a few 
instances has been observed even to divide into two branch-like lobes. The last 
conditions, as well as various other irregular developments, are to be considered as 
abnormal. Base more or less concave, usually with a central cicatrix of attachment 
beyond which it is covered with a concentrically striated and wrinkled epitheca. 
Upper surface celluliferous. Hight of zoarium varying from 5 mm. or less to 50 mm. 
or more; diameter from 10 to over 100 mm. Zocecia with direct, subcircular aper- 
tures, thin walls, those of neighboring cells generally in contact except atthe angles 
of junction, the latter being occupied by angular mesopores of variable though usu- 
ally small size. In the youngest specimens the zocecia are the roundest and the 
interspaces, occupied by the mesopores, the widest, while in the oldest the opposite 
conditions prevail. More or less conspicuous clusters of cells of larger size than 
the average occur at intervals of nearly 4 mm., measuring from center to center. 
Between these cells the mesopores are commonly more numerous than elsewhere, 
and in nearly all cases constitute aggregations of variable extent and substellate 
form. These aggregations are unusually large and conspicuous in the var. orientalis 
to be mentioned presently. In many cases, chiefly old examples, the mesopores 
between the zocecia occupying the intermacular spaces might be overlooked, 
although, as shown by thin sections, they are really numerous even there. Diam- 
etersof an ordinary zocecium about 0.25 mm., with an average of eleven in 3 mm. 
Those forming the clusters vary in size, generally, from 0.25 to 0.88 mm., but in the 
var. orientalis a few in each may attain a diameter of 0.48 mm. Acanthopores 
wanting. 
Internal characters: These, as shown in over one hundred thin sections, are 
very constant in all the essential parts. In tangential sections the form of the 
zocecial tubes varies from perfectly circular to polygonal, their walls in most cases 
being very thin, and the cavity of each intersected by the crescentic edges of one or 
more cystiphragms, The opening left by the cystiphragms is generally lateral and 
of bi-convex shape, occasionally it is subcentrally situated and oval, but more com- 
monly two or more cystiphragms combine to give it a subtriangular form. An 
abnormality is sometimes met with in the confluence of two zocecia (see lower right- 
hand corner of fig. 9). The zocecia are in contact with each other only in part, 
