PAL^ONTOLOG Y. I 2 7 



transverse section through a specimen from the upper Helderberg 

 group, exhibiting the narrow horseshoe-shaped inner walls. The 

 right-hand specimen, found in the same locality, exhibits a side 

 view of the stems, interrupted by constrictions. The vertically 

 intersected central stems have the narrow central area and the 

 lateral carinations of the lamellae well exposed. The right-hand 

 figure in the lower tier of the same plate represents a cluster of 

 stems found in the Hamilton group of Alpena, with well-preserved 

 end cells. 



DIPHYPHYLLUM COLLIGATUM. 



HELIOPHYLLUM COLLIGATUM, Billings. 



Colonies of subparallel stems of the structure of Diphyphyllum, 

 with an internal narrow wall separating the outer finely cellulose 

 from an inner transversely septate area. The growth of these 

 colonies is entirely peculiar; the stems are regularly articulated 

 by deep constrictions, in which constricted parts they are free. In 

 alternation with these constrictions, the calyces become broadly 

 expanded at certain levels, coincident in all the tubes of the colony, 

 and join with their margins under polygonal outlines in a contin- 

 uous floor of astraeiform aspect. At a subsequent period the sur- 

 face of the expanded calyces becomes covered up by an epithecal 

 crust, and from the centre of each of the old calyces a new calyx 

 grows with a contracted base, rapidly dilating above, in order to 

 meet the others again in a common floor, which contractions and 

 expansions follow each other in constant succession. The calyces 

 are radiated by about fifty crenulated lamellse, equal near the 

 margins, but alternately longer and shorter on the sides of the 

 calyces ; the longer ones abut against the narrow inner wall. 

 Found in the upper Helderberg limestones of Michigan, Canada, 

 and at the Falls of the Ohio. 



Plate XXXVHI. — Fig. 3 represents a side view of two silicified 

 fragments from the drift of Ann Arbor. 

 23 



