146 LOWER PENINSULA. 



inches. Some enlarge their diameter rapidly to a certain thickness, 

 and then grow on in a uniformly cylindrical shape ; others are in 

 the young state, slender, flexuose, and irregularly constricted stems, 

 and grow gradually to larger diameters. The surface of the poly- 

 paria is covered by an epitheca with shallow annular wrinkles of 

 growth and longitudinally ribbed by septal strije, which, however, 

 are not in all specimens equally distinct. Calyces spacious, with 

 erect walls, and acute, wedge-like margins ; bottom broad, marginally 

 depressed and flat in the centre. In one place of the circumference 

 the diaphragms are more deeply depressed by a septal fovea. 

 Radial lamellae stout, linear, alternately long and short, but ap- 

 pearing nearly equal on the margins of the calyces, where the sharp 

 crested leaves of the inside expand into low rounded rugae. The 

 extension of the radial crests toward the centre is subject to varia- 

 tions ; in some the central part of the diaphragms remains smooth, 

 and the crests are confined to their peripheral circumference ; 

 in others the crests reach as low carinas to the centre and 'become 

 irregularly entangled in their convergence, but these central por- 

 tions of the crests are merely superficial, and do not intersect the 

 diaphragms to form continuous vertical leaves. The number of 

 lamelltC in calyces of about two and a half inches diameter is 

 150 to 160, half of which are of the smaller size. Found in the 

 upper Helderberg limestones of Michigan, Canada, Ohio, and in 

 the Western States. 



Plate LII. represents specimens of smaller size, found at the 

 Falls of the Ohio. 



ZAPHRENTIS PROLIFICA, Billings. 



Conical, irregularly curved polyp cells, obtusely wrinkled by annu- 

 lar rugce. Middle-sized specimens about three or four centimeters 

 wide at the calyx margin, by a length of from seven to eight centi- 

 meters. Calyces deep, spacious, with erect margins. Bottom, of 

 cells variable ; most frequently its centre is reversed into a laterally 

 compressed, more or less prominent cristiform projection, whose 

 sides are carinated by the centrally uniting radial crests ; in others 

 the bottom is uniformly depressed convex, covered by the en- 

 tangled converging ends of the lamella; ; sometimes also a flat 

 smooth spot is left in the centre of the cell bottom, which merges 



