260 



THE PALEONTOLOGY OF MINNESOTA. 



[McHotrypa <|ucbecensls. 



Mr. Ami's original type is probably a young specimen, being only about 12 mm. in 

 diameter. Zocecial apertures rounded, the largest of those in the clusters attaining 

 a diameter of 0.4 to 0.45 mm., but those occupying the spaces between the clusters 

 average about 0.24 mm., with eleven or twelve in 3 mm. Walls thin. Mesopores of 

 variable size, not isolating the zoo3cia, in most cases readily distinguishable at the 

 surface ; internally with diaphragms averaging about ten in 1 mm. Zooecial tubes 

 intersected by a few oblique curved diaphragms. These are developed chiefly in 

 zones, 3 or 4 mm. apart, in which several occur approximately on the same level in 

 all the tubes. In even these zones the diaphragms are separated but rarely by 

 intervals less than 0.5 mm., while between the zones they may be wanting for a 

 distance of 4 mm., though commonly occurring there at intervals of about 2 mm. 

 Not a sign of acanthopores has been detected on the surface nor in thin sections. 



This species is closely related to M. regularis Foord, sp., differing therefrom 

 chiefly in wanting acanthopores. The same and other differences will be noticed 

 when thin seotions'of it are compared with similar sections of M. infida, M. ivhiteavesi 

 and M. patella, in all of which the zoarium has very nearly the same shape. Young 

 examples are also very much like Prasopora insularis and P. lenticularis. Indeed, it 

 is a matter of no small difficulty to identify with certainty any of the host of dis- 

 coidal Bryozoa that are already known from Lower Silurian deposits without the 

 aid of intelligently prepared thin sections. 



Formation and locality. Rather rare in the Galena Shales at Decorah, Iowa, in shales of the Tren- 

 ton group at Burgin and Danville, Kentucky, and in the Trenton limestone at Trenton Falls, New York. 

 Mr. Ami's type is from rocks supposed to be equivalent at Quebec, Canada. 



Mus. Reg. No. 8128. 



MESOTRYPA DISCOIDEA. n. sp. 



FIG. 16. Mesotrypa discoidea Ulrich, upper beds of the Galena shales, Goodhue county, Minnesota. 

 Collection of E. O. Ulrich. Tangential and vertical sections, x 18, the latter consisting in part of two 

 distinct layers. 



