228 THE PALEONTOLOGY OF MINNESOTA. 



IHomotrypella. 



Genus HOMOTRYPELLA, Ulrich. 



Homntrypella, 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 maculae, 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 zooecial 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 zooecial 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. yracilis 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. 



TremateUa, Hall, 1887, Pal. N. Y., vol. vl, p. 14, Is evidently a synonym of Batostomella, Ulrich, 1882. Jour. Cin. Soc. Nat 

 Hist., vol. v, p. 154. 



