286 



THE PALEONTOLOGY OF MINNESOTA. 



[Diplotrypa limltarls. 



DIPLOTRYPA LIMITARIS, . Sp. 



FIG. 18. Diplotrypa limitarig Ulrich, upper beds of the Galena shales, Goodhue county, Minnesota. 

 Collection of E. O. Ulrich. Transverse and vertical sections, x 18, showing the tabulation of the tubes 

 and the great reduction in the number of mesopores characterizing the species. 



Zoarium, as seen in four specimens, small, hemispherical, 12 mm. or less in diam- 

 eter, and 3 to 7 mm. in hight. Under surface concentrically wrinkled ; upper surface 

 without monticules but presenting rather conspicuous clusters of large cells among 

 which the mesopores are commonly more numerous than elsewhere. Walls very 

 thin ; zooecial apertures polygonal, 0.25 to 0.4 mm. in diameter, with ten to twelve 

 in 3 mm. Mesopores of variable size, not as numerous as the zooscia. Tabulation 

 of tubes comparatively regular and uniform; in the mesopores there are nine or ten 

 diphragms in 0.5 mm., in the zooacial tubes six to nine in 1 mm. As shown at the 

 margin of the vertical section figured above, the tabulation of the mesopores changes 

 suddenly (? always) into that of the zocecia. 



The unusually small number of mesopores distinguishes this species from D. 

 petropolitana and D. westoni (Ulrich). In those species the zooecia are also some- 

 what larger, and the tabulation less uniform. 



Compared with associated and other discoidal Bryozoa, the species of Mesotrypa 

 are separated by their rounded zooacia and much more abundant mesopores, those 

 of Prasopora by the cystiphragms, those of Leptotrypa by their acanthopores, while 

 those of Monotrypa are entirely without mesopores. 



Formation and locality. Upper beds of the Galena shales at localities in Goodhue county, Min- 

 nesota. The best exposure of these beds is in a bluff about thirteen miles south of Cannon Falls. 



