150 LOWER PENINSULA. 



McCoy, called Aulophyllum by Milne-Edwards, I would suggest 

 that a great similarity exists between them and the species under 

 consideration ; but Milne-Edwards asserts the development of an 

 internal wall separating the inner area from the outer, of which in 

 our specimens not the least trace can be observed. Found in the 

 upper Helderberg limestones of Mackinac Island, and at the Falls 

 of the Ohio. 



Plate XL., Lower tier. — The central, shorter, conical specimen 

 is from Mackinac Island. The other specimens are from the Falls 

 of the Ohio. The latter are silicified ; the first is in calcified 

 condition. 



ZAPHRENTIS EXIGUA. 

 Synon., Heliophyllum exiguum, Billings. 



Small, conical, oblique polyp cells, encircled by linear constrictions 

 and intermediate broader rugge. The curved conules are flattened 

 on the longer side and quite convex on the shorter side of the 

 curvature. Diameter of calyces one and a half centimeter in 

 medium-sized specimens, two and a half in the largest ones ; their 

 length is about equal to their width, or a little longer. Calyces 

 moderately deep, surrounded by about seventy alternately large 

 and small lamellce, denticulated at the edges and transversely 

 carinated on the sides by granulose ridges. The lamellae are very 

 stout, protruding with the convexity of their edges above the 

 margin of the outer peripheral walls of the cells. In the median 

 line of the flattened longer side of the cells a conspicuous septal 

 gap is developed, which extends to the centre, dilating into a small 

 horseshoe-formed sinus around which the lamellae are grouped. The 

 centre of the septal gap is occupied by a single lamella, which 

 directly connects with another coming from the opposite side. 

 The lamellas on both sides of the septal gap unite at their central 

 ends in semi-pennate fascicles. 



The interlamellar interstices are all free and open, whereby this 

 form proves itself positively distinct from Cyathophyllum or 

 Heliophyllum, under which name it has been described by Billings. 



Occurs in the upper Helderberg limestones of Mackinac, and in 

 the drift, associated with corniferous limestone fossils ; it is also 



