BRYOZOA. 



Homotrypella mundula.1 



ten to twelve in 3 mm., slightly oblique in young stages, ovate or irregular in shape, 

 in the best preserved examples more or less obscured by the rather large and abund- 

 ant projections of the acanthopores. Interspaces varying, even in small spaces, from 

 contact between the zooecial walls to a width fully equalling the diameter of a zocec- 

 ium. Where widest they are depressed, and in a few instances show the mouths of 

 mesopores, but as a rule these appear as closed solidly. 



Internal characters: In the axial region of vertical sections the tubes sometimes 

 appear to have grown irregularly, and where this is the case they are intersected by a 

 few remote diaphragms. However, in the normally developed straight tubes, these 

 structures are wanting in the axial region except in zones, 2 or 3 mm. apart, extend- 

 ing through the branches. In these each tube has a few. Such a zone is shown in 

 the upper part of figs, b and c. As the tubes turn to enter the peripheral region, dia- 

 phragms become numerous, and in their midst a series of cystiphragms is developed in 

 most of the tubes. The development of the cystiphragms is not uniform, being, as 

 shown at i. c., fig. b, often isolated, while in other cases they may form continuous 

 series of as high as ten or twelve. Such extremes may be noted in a single section, 

 and it seems evident that in the development toward H. gracilis Nicholson, sp., they 

 had by this time lost some of their importance and were gradually being dropped. 

 The mesopores are inconspicuous features in vertical sections, being filled almost 

 solidly with sclerenchyma in which their diaphragms are but illy distinguishable. 



In tangential sections the zocecia commonly present a dark ring-like investment, 

 in which the sharply defined acanthopores are mostly situated. The interspaces are 

 of a lighter color, and but rarely exhibit any positively defined mesopores. Cysti- 

 phragms, so far as observed, may be detected in perhaps half of the zooecia. 



A closely related but smaller species than H. granulifera (Chcetetes granuliferus 

 Ulrich) from the Trenton of Kentucky. In its general aspect it is exceedingly like 

 H. gracilis Nicholson, sp., of the Hudson River rocks, and it is almost certain that 

 the two represent stages in one line of development. However, comparative abund- 

 ance of cystiphragms in the present species will distinguish them at once, these 

 structures being of very rare occurrence in H. gracilis. In the latter the acantho- 

 pores also are smaller, so that the surface granulation is less coarse, 



Formation and locality. Galena shales. Common at Decorah, Iowa; rare in the vicinity of Cannon 

 Falls, Minnesota. 



Mus. Reg. No. 80SO. 



