1 8 Cincinnati Society of Natural History. 



gins of calices or forming apparently closed tubercles. Corallites 

 springing from both sides of median axis, forming two laminae, 

 sometimes marked by a calcareous membrane; oblique and thin 

 walled at first, but soon bending outwards and proceeding straight 

 to the surface, the walls there moderately thickened. 



Obs. — This species grows in much the same manner 2.% dawsom 

 and maiiDindata. It is distinguished from the former by the 

 smoother surface, and from the latter by this, as well as the fact 

 that the corallites pass from the separating membrane directly to 

 the surface. The calcareous plate is sometimes so well defined 

 that a specimen may be fractured along it and separated into two 

 halves. One of us has specimens in this condition. 



The identy of decipiens, Rom., with />-^/;^^jr^, D'Orb., has been 

 denied by some. We adopt the view of Dr. Nicholson, and con- 

 sider it as a synonym. Some have also claimed that frondosa is 

 one of the forms generally taken as mammulata. 



Formation and Location. — Lovver Silurian Cincinnati (iioup. 

 Cincinnati, O. 



39. — M. PAVONi.A, l)'(_)rbigny. 



Ptilodictya pavonia, D'Orb. Prodr. de Paleont., Vol. I, p. 22, 

 1850. 



Chcetetes pavonia, Ed. iSc Haime. Poly. Foss. des Terr. Pal. 

 p. 267, 1851: Rominger. Proc. Acad. Nat. Sci , Phila., p. 116. 

 1866. 



Cyclopora jaiiicsi, Prout. Trans. Acad. Sci., St. Louis. Vol. I, 

 p. 578, i860. 



Stictopora clathratiila, James. Cat. Foss. Cin. (ir. (named but 

 not figured or described), 1871. 



Chcrtctcs clathratuliis, James. Nicholson, Quart. Jour. Geol. 

 Soc, XXX, p. 259, 1874; Pal. Ohio, IF p. 209, 1875. Ann. Nnt. 

 Hist., ser. 4, XVIII, p. 91, 1876. 



Heterodictya pavonia, Ulrich. Cat. Foss. Cin. (jr. (named but 

 not figured or described), p. 10, 1880. 



Corallum forming a thin, undulating e.xpansion. often of con- 

 siderable extent, varying in thickness from one to about two lines, 

 the corallites in two layers with their bases fixed to a medium plane 

 marked by a delicate membrane and opening on opposite sides of 

 the corallum. Surface often with low, rounded monticules, often 

 obscure, and arranged in diagonal rows at intervals of from one to 

 one and a half lines apart, occupied by calices of ordinary size. 



