lO LOWER PENINSULA. 



HELIOLITES MEGASTOMA, McCoy. 



Visceral tubes about two millimeters wide. Orifices crenulated 

 by a cycle of twelve low vertical crests. Diaphragms flat, closely 

 set. Coenenchym tubules about half a millimeter wide. Interstitial 

 spaces filled by them, subject to many variations in different speci- 

 mens ; in some the distance between the larger circular tubes is 

 smaller than their own diameter, in others it is larger. External 

 mode of growth convex, or in subplane expansions, covered by a 

 concentrically wrinkled epitheca on the impressed lower concave 

 side. It is a common species in the Niagara group of Drummond's 

 Island, Point Detour, and many other localities ; in the drift depos- 

 its of the Lower Peninsula it also frequently occurs. The Niagara 

 group of Indiana, Iowa, Kentucky, etc., and the Silurian strata of 

 Bohemia inclose forms perfectly identical with those of Michigan. 



Plate I. — Fig. 3 represents a silicified specimen from Point 

 Detour, natural size, the structural details of which will all be seen 

 more distinctly by using a weak magnifier, when looking at the figure. 



HELIOLITES PYRIFORMIS, Hall. 



Tubes about one millimeter wide, radiated by a cycle of twelve 

 spinulose, vertical crests. Diaphragms flat, simple, or anchylosed 

 with the spinulose, vertical crests into an irregularly cellulose mass, 

 filling the tube cavities. Coenenchym of minutely tubular struc- 

 ture, divided by transverse septa. Interstitial spaces between the 

 larger tubes equal to or larger than one tube diameter. External 

 growth, subglobular. 



The mode by which the specimens are preserved alters their ap- 

 pearance considerably. Some of the specimens, with simple trans- 

 verse diaphragms exposed to weathering, are apt to have the dia- 

 phragms of the larger tubes destroyed, and the channels of the 

 larger tubes are seen all open, surrounded by the well-preserved 

 mass of the smaller coenenchym tubules. In other specimens where 

 the vertical crests, intermingling with the diaphragms, give the larger 

 tubes more strength, the coenenchym is found destroyed and the 

 larger tubes are preserved, and present themselves as free cylindri- 

 cal, longitudinally carinated columns. Still other specimens are 



