PALAEONTOLOGY. 1 3 



Plasmopora and Heliolites are in structure so similar that it 

 often becomes difficult to make a distinction between the two 

 forms. 



PLASMOPORA FOLLIS, Milne-Edwards. 



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

 with their circular crenulated orifices by interstitial spaces equal to a 

 tube diameter or smaller. Vertical laminae of the coenenchym often 

 holding aposition 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 laminae 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. 



PLASMOPORA ELEGANS, Hall. 



Synon., Heliolites elegans, Hall. 



Heliol. spinipora. Hall. 

 Plasmopora scita (?) Milne-Edwards. 



Tubes a little over half a millimeter wide, radiated by twelve prom- 

 inent longitudinal rows of spinules obliquely directed upward, and 

 nearly reaching the centre. Diaphragms complicated and anchy- 



