12 LOWER PENINSULA. 



condition. Specimens from the Silurian strata of Bohemia perfectly 

 correspond with the American form. 



HELIOLITES SUBTUBULATUS (?) McCoy. 



Visceral tubes only half a millimeter wide, with twelve delicate 

 marginal crenulations around the orifices. Coenenchym tubular, 

 very minute. Interstitial spaces between the larger tubes broad. 

 Transverse diaphragms simple, regular, disposed at close distances. 



External growth in convex, rounded masses. I have identified 

 this form with McCoy's species on account of a general resemblance 

 to his figures, but had no opportunity to compare specimens. It 

 occurs in the Niagara group of Point Detour, Drummond's Island, 

 etc. ; also in Iowa, at Masonville. 



Plate I. — Fig. 4, lower specimen found at Marblehead, of Drum- 

 mond's Island, natural size ; upper specimen from Masonville, 

 Iowa. 



PLASMOPORA. 



Milne-Edwards. 



Visceral tubes circular, not projecting above the general sur- 

 face, radiated by twelve well-developed vertical rows of spinulose 

 projections. Coenenchym formed of stout vertical laminre, which 

 partly appear as the extraneous continuation of the radial crests 

 'of the tubes. They intersect each other in irregular manner, in- 

 closing tubular spaces between, which are transversely septate, like 

 the tubules of the coenenchym of Heliolites ; but while in the latter 

 they are actually closed tube channels, in the coenenchym of Plas- 

 mopora no closed tube walls seem to exist, ^ but_oi'ily lac_uiise(in- 

 jttci^c^ i-n f^Sfi ..^ tersecting each othc^betwcen the vertical plates, 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. 



J- f «»j 



