SUB-GENUS MONOTRYPA: 195 



possesses two sets of corallites, which differ not only in size, 

 but also in their tabulation. 



Hoi'izou and Locality. — Cincinnati Group, Ohio. An out- 

 wardly similar form occurs in the Hudson River Group of 

 Canada ; but not having prepared thin sections of this, I am 

 not certain of its identity with the Cincinnati type. 



Monticulipora (Monotrypa) pavonia, D'Oibigny. 



(Fig. 41, and PI. VI. figs. 3, 3^.) 



Ptilodidya pai'onia, D'Orbigny, Prodr. de Paleont., vol. i. p. 22, 1850. 

 Chcetetes pavonia, Milne- Edwards and Haime, Pol. Foss. des Terr. Pal., \). 

 267, PI. XIX. figs. 4, 4^7, 1 85 1. 

 ,, ,, Rominger, Proc. Acad. Nat. Sci. Phil., 1866, p. 116. 



Stidopora clatJiratiila, James, Cat. Foss. Cincinnati Group, 1871. 

 ChcEtdes'i dafliratidus, Nicholson, Quart. Journ. Geol. Soc, vol. xxx. p. 509, 



PI. XXX. figs. 1-1/7, 1874. Pal. Ohio, vol. ii. p. 209, PI. XXII. figs. 



2, 2h, 1875. Ann. Nat. Hist., ser. 4, vol. xviii. p. 91, PL V. figs. 9, 9^, 



1876. 

 Heterodidya pavonia, Ulrich, Cat. Foss. Cincinnati Group, p. 10, 1880, 



Spec. Char. — Corallum forming a thin undulating expansion, 

 often of considerable extent, and varying from half a line to 

 about two lines in thickness. The expansion grew in an erect 

 position, and it consists of two layers of corallites, which have 

 their bases fixed to a median plane marked by a delicate 

 undulating calcareous membrane, and which open on opposite 

 sides of the corallum. The corallites vary from a quarter of 

 a line to a line in total length, and though oblique at their 

 origin, they almost immediately bend outwards, so as to open 

 nearly at right angles to the general surface, or with only a 

 slight obliquity to it. The calices have the form of elongated 

 pentagons, with rounded angles, their long diameter being 

 from i-iiothto i -90th inch. Their size is tolerably uniform, 

 and there are no minute interstitial tubes. The calices are 

 often arranged in tolerably regular obliquely intersecting 

 lines ; and the surface shows low rounded elevations, which 

 are often obscurely marked, are arranged in diagonal rows 



