SUB-GENUS MONOTRYPA. 199 



thick-walled in the outer portion of their course, and are to a 

 large extent fused with one another, though the original lines 

 of demarcation between adjoining tubes can still be recognised. 

 Centrally, the corallites are thin-walled and polygonal. Tabulae, 

 which are always complete and approximately horizontal, are 

 present both in the axial and the peripheral portions of the 

 corallum, but are most abundantly developed at the point 

 where the corallites begin to bend outwards to reach the 

 external surface. 



Obs. — The peculiar form and mode of growth of the coral- 

 lum of this species (PI. II. fig. 5), taken alone, would render its 

 distinctness highly probable ; and its internal characters are 

 likewise quite peculiar. Tangential sections (PI. II. figs. 5^ 

 and 5<^) show that the corallites are very uniform in point of 

 size (about i-8oth inch in diameter), and are exclusively (so 

 far as I have seen) of one kind. The original polygonal walls 

 of the tubes can still be readily recognised ; but their internal 

 cavities are much contracted by a thick deposit of sclerenchyma, 

 by which their shape is, on the whole, rendered oval or circu- 

 lar. No " spiniform corallites " are developed. Axial sections, 

 whether longitudinal or transverse, show that the corallites in 

 this region are thin-walled and polygonal in shape, the thick- 

 ening of the corallites only commencing when they begin to 

 bend outwards. Longitudinal sections (PI. II. fig. 5^:) further 

 show that there is no difference in internal structure in any of 

 the corallites, and that all alike are furnished with a moder- 

 ate number of horizontal and complete tabulae, which become 

 somewhat fewer in the axis of the stems. 



I know of no species of Monticulipora with which it is nec- 

 essary to compare M. briarea closely, both its external and 

 internal characters being quite peculiar. The specimen from 

 which the foregoing description was mainly drawn up, was 

 kindly presented to me by Mr U. P. James of Cincinnati, to 

 whom the first recognition of the form as new is really due. 



Horizon and Locality. — Cincinnati Group, near Cincinnati, 

 Ohio. 



