220 THE PALEONTOLOGY OF MINNESOTA. 



[Montlcullpora arborea. 



of the figure is taken from the lower part of the section where the transverse parti- 

 tions are less closely arranged than near the surface of the zoarium. The walls have 

 a lineo-granose structure. Tangential sections show that the zooecial tubes are 

 polygonal and thin-walled, the opening left by the cystiphragms ovate or sub- 

 ciicular and situated laterally or subcentrally. At the angles of junction the walls 

 are slightly thickened, and there is some evidence to show that very small acantho- 

 pores were developed at these points. 



This fine species has an external resemblance to irregular examples of M. Icevis 

 Ulrich, from the Cincinnati group of Ohio, but the zooecia and internal structure of 

 M. grandis are so much larger and different that the relation between the two species 

 must really be quite remote. Excepting that mesopores are wanting entirely, the 

 interior, as brought out by thin sections, is very similar to Prasopora contigua Ulrich, 

 and I have considered the advisability of referring the species to Prasopora rather 

 than Monticulipora. For the present, however, we must conclude that the absence 

 of mesopores and lobate-massive instead of discoid growth are features sufficient to 

 disbar the species from Prasopora. 



Formation and locality, At the base of the Trenton shales and the top of the underlying limestone 

 at Minneapolis and Cannon Falls, Minnesota. Vertical range apparently very limited. 



Mus. Reg. No. 5969. 



MONTICULIPORA ARBOREA, H. Sp. 

 PLATE XX. FIGS. I-D and 13. 14. 



Zoarium dendroid, rising to a hight of 4 or 5 cm.; branches subcylindrical, 

 dividing at intervals varing between 5 and 20 mm.; surface with low monticules, or 

 smooth. Zooecial apertures small, subangular, enclosed by rather thick, minutely 

 granulose walls, on which small acanthopores, one or two to each zocecium, are to be 

 distinguished from the numerous small granules, which, together with the acantho- 

 pores, are quickly removed when exposed to the weather. About fourteen of the 

 cells between those occupying the monticules or those in the clusters occur in 3 mm. 

 Mesopores wanting. 



Internal characters: The tabulation of the zocecial tubes is compact throughout, 

 and very much so in the peripheral region. The tubes bend outward in a gradual 

 curve from the imaginary axis, and have cystiphragms from the beginning. In the 

 axial region these are large and about two-thirds of the tube-diameter apart, becom- 

 ing gradually a little nearer to each other until the fully matured condition of the 

 species is reached, when they recur at intervals scarcely equalling one-fourth of the 

 diameter of the tube. They also change their shape here, the opening which had 



