258 THE PALEONTOLOGY OP MINNESOTA. 



[Mesotrypa. 



Mbsotrypa infida Ulrich. 



PLATE XVII, FIGS. 1-8. 



Diplotrypa infida Ulrich, 1886, Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 88. 



Zoarium discoid or subhemispheric, with the base fiat or concave and partly 

 covered with a concentrically wrinkled epitheca ; or it may be parasitic and conform 

 with the shape of the body grown upon ; hi^ht or thickness from 2 to 7 mm., diame- 

 ter from 12 to 20 mm. Zooecial apertures varying from polygonal to subcircular, 

 the shape depending upon the number and size of the mesopores. In some speci- 

 mens the latter are almost certain to be overlooked, the zooecia being angular and 

 seemingly in perfect contiguity (pi. XVI f, fig. 8). In others they are large enough 

 to constitute an obvious external feature (fig. 7). The latter condition is to be 

 regarded as less mature than the former, since in it the acanthopores are scarcely 

 distinguishable, while they are readily made out where the mesopores are smallest. 

 At intervals of 3 or 4 mm. there are clusters of large zooecia varying in diameter 

 from the smaller or ordinary sizes of 0.2 to 0.24 mm. to 0.4 or 0,45 mm. An average 

 of eleven of the ordinary size in 3 mm. 



Internal structure: In vertical sections the tubes are everywhere perpendicular 

 to the basal membrance. Their walls are a little thicker than usual in species of 

 the genus. In the lower part of the zoarium the mesopores with their numerous 

 diaphragms are very conspicuous. In following them upward they seem to be re- 

 duced in diameter, permitting the zooecial tubes to come in contact with each other. 

 These zones may be repeated several times. Diaphragms are numerous though not 

 regularly distributed in the zooecial tubes. The appearances presented by them are 

 exceedingly variable. Some appear to be horizontal and straight or slightly bowed 

 down in the middle (figs. 4 and 6), others are obliquely curved and perhaps overlap- 

 ping (fig. 3), while still others are f annel-shaped (fig. 5). This variability however 

 is not a structural peculiarity, but is due to the varying angles at which the dia- 

 phragms are cut by the section. 



Tangential sections vary according to the .depth from the surface at which they 

 divide the zoarium. In the immature region (fig. 2) we have rounded zooecia, almost 

 completely surrounded by mesopores, and small acanthopores. In the mature region 

 (fig. 1) the mesopores are much smaller, the zooecia somewhat larger and subangular, 

 and the acanthopores larger. In most of the zooecial cavities the oblique and some- 

 what funnel-shaped diaphragms are represented by curved lines, often closely sim- 

 ulating the appearances of ordinary cystiphragms. 



