PAL^ONTOLOG Y. 



139 



and almost vanishing near the bottom. The arrangement of the 

 plications into four fascicles with intermediate gaps is in this 

 species particularly plain, and the apertural gap situated in the 

 centre of the large curvature of the horn-shaped cells is the largest. 

 In vertical sections the structure is found to be perfectly con- 

 formable with Cystiphyllum, composed of incased vesiculose cups. 

 There are two varieties : in one the plications expand toward the 

 margin of the cups into broad bands decorated with granulations ; 

 in the other form the plicae split toward the margin into narrower 

 fine striae. The latter variety, which has also less obliquely erect 

 cups, is found in the upper Helderberg limestones of the Falls of 

 the Ohio ; the other is found in the drift of Ann Arbor, on Mackinac 

 Island, and in the corniferous limestone of Port Colborne, in 

 Canada, 



Plate L., Lower row. — The |^left-hand figure represents silicified 

 specimens from Port Colborne. Various other forms of the genus 

 Cystiphyllum occur at the Falls of the Ohio, and at Columbus, 

 Ohio, which I omit to describe, as not found in Michigan. 



ZAPHRENTIS, Rafinesque. 



Inclusive of Streptelasma, Hall, as a sub-generic fonn. 



Simple conical polyparia, composed of a series of invaginated, 

 radially plicated cell cups. The superimposed cell cups are with 

 their side walls so intimately united, that no interstice is left be- 

 tween them, and all combine into one simple compact wall folded 

 into laminar plications. The interstices between the plications 

 are, within the terminal cups, free and open throughout. The 

 plications of all the incased cups are connected into continuous, 

 strong vertical leaves intersecting the whole length of the corallum. 

 The bottom part of each cup remains free, and is separated from 

 the subjacent one by a small interstice. It appears in vertical sec- 

 tions through the corallum as a transverse laminar diaphragm. 

 These superimposed diaphragms extend from wall to wall ; their 

 peripheral margins are deflected and connect with the outer wall at 

 an acute, downwardly directed angle. The interlamellar interstices, 

 which are open in the end cell, become, below its bottom, in- 



