306 THE PALEONTOLOGY OF MINNESOTA. 



LMonotrypa nodosa. 



destroyed during the process of fossilization. But the study of new material, and 

 other sets of thin sections shows that this explanation Is inadequate, since, while 

 the diaphragms are always absent in this form they are present in all associated 

 Bryozoa ; and it stands to reason that the conditions under which the two sets of 

 specimens were preserved must necessarily have been identical. I am obliged there- 

 fore to consider their absence in the Monotrypa as normal, and to give these peculiar 

 Minnesota specimens a new name. 



M, intabulata agrees with M. rectimuralis* not only in the external appearance of 

 the zoarium but in having straight walls and minute triangular open spaces at the 

 angles of junction as well. As differences we have diaphragms two to four times 

 their diameter apart, and rather conspicuous clusters of large cells in the latter and 

 none in the former. 



Among associated forms only Monticulipora grandis and Bythotrypa laxata have 

 a similar growth, the species of Prasopora and Mesotrypa forming discoidal zoaria. 

 Both of these species however are too widely different structurally from Monotrypa 

 to be confounded. 



Formation and locality. Not uncommon in the upper part of the Galena shales (Fusispira beds) 

 at several localities in Goodhue and Fillmore counties, Minnesota. 



Mus. Reg. No. 8379. 



MONOTRYPA NODOSA, n. sp. 



(Not Figured.) 



Monticulipora (?) ortoni WHITFIKLD, 1882. Geol. Wis., vol. iv, p. 251. (Not Chcetetes ortoni NICHOL- 

 SON, 1875. Pal. Ohio, vol. ii, p. 211.) 



Zoarium commonly beginning its growth upon shells over and beyond which 

 it spreads till it forms large discoidal or hemispheric masses, often over 5 or 6 cm. in 

 diameter. Sometimes the masses are shapeless, but as a rule the base is concave, 

 and, where it projects beyond the covered shell, clothed with a wrinkled epitheca. 

 Upper surface with more or less prominent subconical monticules, averaging about 

 3.4 mm. from summit to summit. Zooacia with thin walls, polygonal and rather 

 regularly arranged apertures, nine or ten in 3 mm.; apertures occupying the mon- 

 ticules but little if at all larger than those in. the intermediate spaces. Summits of 

 monticules occasionally appearing solid. Not a trace of either mesopores or acan- 

 thopores has been observed. Internal characters unknown, none of the specimens 

 seen being fit for sectioning. 



*In the original description of the species I included, erroneously, a hemispheric or lenticular, tuberculated form tliat is 

 very common In the Hudson river rocks at Savannah, Illinois, and Delaflelcl and Iron Ridge, Wisconsin. As it may justly be 

 expected to occur in the southern part of Minnesota, the next brief description of its known characters may be of advantage 

 to students of the palaeontology of the State. 



