40 



LOWER PENINSULA. 



observed, and the tube channels are found to be almost smooth ; 

 this is not in all cases owing to a want of development of the crests 

 or spinules ; these seem often to have been obliterated by im- 

 perfect preservation in the process of petrification. 



Diaphragms somewhat distant and oblique. Pores large, margi- 

 nal, causing a pouch-like dilatation of the tube wall at the spot 

 •where situated. Diameter of tubes in the wider transverse direction 

 varies in different specimens, from a half to one millimeter, which 

 difference in size greatly alters their aspect. The degree of com- 

 pression of the tubes, their more erect or more prostrate position in 

 various specimens also cause numerous variations in their appear- 

 ance, but no tangible line between one and another of the forms 

 exists. I have, for this reason, considered all of them as repre- 

 senting the modifications of one and the same species. It occurs 

 in great abundance in the Niagara group of Drummond's Island, 

 at Point Detour, and in other localities. It is often found also in 

 the drift of the Lower Peninsula. The specimens are all silicified, 

 and in but few of them are the more delicate structural characters 

 well preserved. Certain specimens found in the drift exhibit prin- 

 cipally the silicified casts of the tube channels, which have the form 

 of flattened bands with rounded mamiform protrusions and inter- 

 mediate indentations of the lateral margins ; on the summit of each 

 protrusion a pore channel or its cast is situated. 



Plate XVI. — Fig. i represents a silicified specimen from the drift 

 of Ann Arbor, Fig. 2 is found at Drummond's Island. The latter 

 specimen exhibits the longitudinal rows of spinules very distinctly, 

 but the object is too small to be properly seen in the not magni- 



fied figure. 



