228 THE PALEONTOLOGY OF MINNESOTA. 
{Homotrypella. 
Genus HOMOTRYPELLA, Ulnich. 
Homotrypella, ULRICH, 1886. Fourteenth Ann. Rep. Geol. and Nat. Hist. Surv. Minn., p. 83. 
Zoarium somewhat irregularly ramose, occasionally palmate or frondescent ; 
monticules wanting, but small macule, consisting of clusters of mesopores, often 
present. Zocecia with rounded apertures, the latter sometimes inflected by the acan- 
thopores. Mesopores small, abundant, in some cases completely isolating the zocecia. 
Acanthopores abundant, of medium size, generally imparting to the surface a min- 
utely granulose character. Cystiphragms developed chiefly in the median region 
of the zocecial tubes, being absent usually just beneath the surface and never present 
in the axial region. 
Type: H. instabilis Ulrich. 
This genus was established for the reception of a small but eminently natural 
group of Lower Silurian species that could not be included in any of the other 
genera of the family. Since then other forms have been discovered, and the classi- 
fication of several others changed, so that now no less than eleven, perhaps twelve, 
species of the genus are known to me. These range from the Birdseye to the top of 
the Lower Silurian, each of the more important subdivisions containing one or more 
species. 
In the ramose habit of growth the genus resembles Homotrypa Ulrich, but the 
abundant mesopores are a distinguishing mark of some importance. A comparison 
with Peronopora Nicholson, and Atactoporella Ulrich, shows the following differences : 
In the first the zoaria are bifoliate, in the second usually parasitic, and in both the 
cystiphragms are developed in an almost uninturrupted series throughout the length 
of the zocecial tubes. 
Fuller investigations into the affinities of these fossils have shown good grounds 
for redistributing the species heretofore referred to Batostomella. That genus must, 
therefore, be restricted to the Devonian and Carboniferous species originally 
intended as types.* This leaves the Lower Silurian species unplaced generically. 
Since large specimens of B. gracilis Nicholson, and many of the ordinary forms of 
B. meeki James, sp., from the Cincinnati group of Ohio, often have a few cystiphragms 
developed in the curve of the tubes, and as their other characters are in no wise 
strongly opposed to a union with Homotrypella, it seems best, at any rate provision- 
ally, to place them here. However, the B. simulatrix Ulrich, group of species cannot 
be admitted, and to accommodate them a new generic name will have to be proposed. 
*Trematella, Hall, 1887, Pal. N. Y., vol. vi, p. 14, is evidently a synonym of Batostomella, Ulrich, 1882, Jour. Cin, Soe, Nat 
Hist., vol. v, p. 154. 
