12 LOWER PENINSULA. 



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 

 found, in which, by incrustation of the vv^all substance, the tube 

 channels are narrowed, and the contrast between the larger and 

 smaller tubes is diminished ; these have a spongious aspect, and ap- 

 pear at first glance to be a fossil of totally different structure from 

 the more regularly formed specimens. It is sometimes difficult to 

 distinguish this species from the next described species, Heliolites 

 intcrstinctiis. This species is found in association with the former 

 kind at Drummond's Island, and in the other mentioned localities 

 in Michigan ; it is likewise common in the drift. The Niagara 

 group of Iowa and Wisconsin incloses the same form. 



Plate I. — Fig. 2 represents two specimens from Drummond's 

 Island, in natural size. 



HELIOLITES INTERSTINCTUS, Linn. 



Visceral tubes from one to one and a half millimeter in width. 

 Vertical crests quite prominent, almost reaching the centre, and 

 composed of rows of spinules pointing obliquely upward with their 

 apices. Coenenchym composed of minute, polygonal, transversely 

 septate tubules. Interstitial spaces between the larger tubes usu- 

 ally much exceeding one tube diameter. Diaphragms rarely flat, 

 and simple, generally complicated into a cellulose network with the 

 spTinulose, vertical crests, with a nodular projection in the centre, 

 formed by the converging apices of the spinules. No central col- 

 umella. In vertical sections the channels of the larger tubes are 

 scarcely distinguishable from the surrounding septate coenenchym, 

 because the intersection of the spinules with the diaphragms divides 

 the interior of the larger tubes into small cell spaces similar to the 

 surrounding coenenchym tissue. The visceral tubes always resist 

 decay better than the coenenchym, and are preserved as slender, 

 longitudinally carinated columns, held together by a portion of un- 

 destroyed coenenchym. The mode of growth is in discoid, subplane 

 expansions, with a concentrically wrinkled epithecal crust on the 

 lower side. 



