14 LOWER PENINSULA. 



tween the vertical plates intersecting each other. Diaphragms of 

 visceral tubes numerous, flat, but usually complicated by intersec- 

 tion with the vertical rows of spinules almost reaching to the 

 centre. 



The coenenchym usually resists weathering better than the 

 tubes — large, open channels being found to occupy the places of 

 the latter in such specimens. 



Plasmopora and Heliolites are in structure so similar that it 

 often becomes difificult to make a distinction between the two 

 forms. 



PLASMOPORA FOLLIS, Milne-Edwards. 



Tubes from one to one and a half millimeter in diameter, separated 

 at their circular crenulated orifices by interstitial spaces equal to a 

 tube diameter or smaller. Vertical laminae of the coenenchym often 

 holding a position toward the tubes as of external radii, and inclosing, 

 by mutual junction and intersection by transverse dissepiments, ver- 

 tical rows of cell spaces very similar to the coenenchym of Helio- 

 lites ; but the vertical walls of these septate pseudo tubules are 

 much stouter than the tubular walls in Heliolites, and comparative- 

 ly much stouter than the transverse septal laminse dividing them. 

 Grows in pyriform or subcylindrical, club-shaped masses of mod- 

 erate size, not often exceeding that of a man's fist ; the conical base 

 is attached and exhibits a rudimentary epithecal crust ; in the 

 subcylindrical specimens, the coenenchym cells and orifices on the 

 side faces of the stems are usually closed, and only those of the 

 convex terminal disk apert. Rarely found in the Niagara group 

 of Point Detour, and sometimes in the drift of Michigan. It is a 

 very common species, however, in the Niagara group of Indiana, 

 Kentucky, and Tennessee. 



Plate HI. — Fig. 2 represents two species of Plasmopora, arranged 

 in a group to economize space. The two upper figures are small 

 silicified specimens of Plasmopora follis ; the elongated one was 

 found at Paul's Station, Indiana ; the other, of rounded form, is 

 from Charleston Landing, Indiana. 



