BRYOZOA. 239 
Homotrypa subramosa.] 
later Homotrypella (?) ovata, ‘illustrated on plate XVIII. The mesopores are on the 
whole smaller and do not separate the zoccia so completely, and sometimes the dif- 
ference in these respects is greater than is shown in the illustrations. The difference 
between vertical sections of the two forms is better marked, the cystiphragms being 
more numerous and diaphragms wanting in the axial region in the present species. 
Homotrypa exilis is another closely allied form, but differs in the opposite manner, 
the mesopores being fewer (see plate XIX). 
This is an important stage in the line of development discussed on page 216. 
Formation and locality.—Middle third of the Trenton shales at St. Paul and Minneapolis, Minnesota. 
Homotrypa supramosa Ulrich. 
PLATE XIX, FIGS. 21-28. 
Homotrypa subramosa ULRICH, 1886. Fourteenth Rep. Geol. Nat. Hist. Sur. Minn., p. 81. 
Homotrypa insignis ULRICH, 1886. Fourteenth Rep. Geol. Nat. Hist. Sur. Minn., p. 82. 
Zoarium subramose, frequently though irregularly divided ; branches compressed 
or subcylindrical, their extremities often bulbous. Size of branches varying greatly, 
the smallest 4 or 5 mm. in diameter, the largest 6 to 9 mm. thick, and as much as 
25 mm. wide. Average specimens are about 6 mm. thick and between 8 and 12 mm. 
wide, with the total hight of zoarium rarely exceeding 60 mm. Surface without 
monticules, nor are the clusters of large cells very conspicuous. Zocecia with rather 
thin walls and polygonal, direct apertures ; twelve to fourteen in 3mm. Zoccial 
apertures shallow, exposing the cystiphragms when in a good state of preservation. 
These structures leave but a small opening, and when the fossil has suffered a little 
from attrition (a frequent occurrence in the beds holding the species most abun- 
dantly) in which case the true walls are obscured or cut away, the appearance is very 
deceptive, the apertures seeming to be very small and oblique, and much the greater 
part of the surface occupied by wall-substance. Acanthopores varying in number 
and size, sometimes as numerous as two to each zocecium. More commonly the 
number is little more than half that extreme. In many cases they are large enough 
to constitute a marked external feature. In others, however, apparently in an 
equally good state of preservation, they are so small that it is difficult to detect 
them even with the aid of a good lens. 
Internal characters: Vertical sections show that the tubes proceed in a gradually 
increasing curve from the axial region outward to the peripheral region, in which 
they are approximately at right angles to the surface ; that in the axial region the 
tubes are rather large, with wavy walls, and crossed by straight or oblique dia- 
phragms, either in zones or occurring at intervals varying from one to three times 
