SUB-GENUS MONOTRYPA. 193 



Monticulipora (Monotrypa) discoidea, James. 

 (PI. IV. figs. 3, 3/) 



Chatdcs discoidms, James, Cat. Foss. Cincinnati Group, 1871. (Named, but 



not figured or described.) 

 Chceteies discoideus,^\ckvQ)\?,ox\, Quart. Journ. Geol. Soc, vol. xxx. p, 511, 



PI. XXX. figs. 4-4^/, 1874. Pal. Ohio, vol. ii. p. 206, PI. XXI. figs. 15- 



15^, 1875. Ann. Nat. Hist, sen 4, vol. xviii. p. 88, PL V. figs. 7, 7^, 



1876. 



Spec. Char. — Carallum free, discoid, concavo-convex or 

 plano-convex, sharp-edged, from five to eight Hnes in diameter, 

 and about one-tenth of an inch in thickness in the centre. 

 Under surface more or less concave, as a rule, covered with 

 a thin, smooth, and irregularly striated epitheca, which usually 

 exhibits two or three marked concentric wrinkles. Upper 

 surface carrying the calices, gently convex, and not exhibit- 

 ing any monticules. Calices polygonal, thin-walled, sub-equal, 

 mostly about i-8oth inch in diameter. There is no proper series 

 of small corallites ; but there is a largely developed series of 

 thick-walled hollow tubuli (" spiniform corallites "), one being 

 usually situated at each angle of junction of adjacent tubes. 

 The corallites are directed at right angles to the basal plate, 

 being thin-walled at first, but ultimately becoming moderately 

 thickened ; and the visceral chambers are crossed by a mod- 

 erate number of complete horizontal tabulae, which are often 

 placed at corresponding levels in adjacent tubes. 



Obs. — The external characters and mode of growth of M. 

 discoidea, James, are sufficiently alluded to in the preceding 

 description (PI. IV. figs. 3-3^), and need not be further re- 

 ferred to here. The internal structure is highly characteristic, 

 and places the validity of the species beyond doubt. 



Tangential sections taken just below the upper surface 

 show that the ordinary corallites (PI. IV. figs, y and 'x^d) are 

 polygonal in shape, tolerably equal in size (about i-8oth inch 

 in their longest diameter), and moderately thick-walled, the 

 lines of demarcation between adjacent tubes remaining dis- 



N 



