SUB-GENUS MONOTRYPA. 



191 



cases, there may be two of these prominences ; and, whether 

 single or double, the superior elevation may project above the 

 base to a considerable height as compared with its width at 

 the base. The maximum height varies from three lines to 

 nearly one inch. The upper surface is covered with the 

 calices, which are thin-walled, polygonal, and approximately 

 equal in size, varying from about i-8oth inch to i-6oth inch 

 in diameter. There are no small interstitial corallites of any 

 kind ; but the surface shows numerous clusters of tubes which 

 are slightly larger than the average, and which are very 

 slightly elevated above the general level. The walls of all 

 the corallites are thin and delicate, no thickening taking place 

 as the surface is approached. The tabulae are numerous, com- 

 plete, equally developed throughout the entire length of the 

 corallites, straight or slightly curved, and mostly from i-iooth 

 to I -90th inch apart. 



Obs. — This form from its general aspect would be set down 

 as a near relation oi Montictilipora petropolitana, Pand,, from 



Fig. 40. — A, A specimen of MonticiiUpora petasifonnis, Nich., of the natural size, viewed 

 from above, and showing the prominence of the central part of the disc ; B, Tangential 

 section of the same, enlarged eighteen times, showing the absence of interstitial tubes 

 and the delicate character of the walls ; c, Vertical section of the same, enlarged eighteen 

 times, showing the thin walls and the numerous and equally developed tabuice. Cincin- 

 nati Group of Ohio. 



which, however, it is even superficially separable by its peculiar 

 and seemingly constant form (fig. 40, a). In its minute struc- 

 ture the present species is quite distinct from Af. petropolitana, 



