SUB-GENUS HETEROTRYPA. 



135 



in the thickened peripheral portion of the tubes than centrally ; 

 while the interstitial tubes are more closely tabulate than the 

 average corallites. 



Obs. — As far as external characters go, this species is dis- 

 tinguished by its flattened, sub-frondescent form, and by the 

 presence of numerous star-shaped " maculae," formed by clusters 

 of small corallites, and not elevated above the general surface 

 (PI; V. figs. 2, 2a). 



As concerns its minute structure, tangential sections (fig. 

 23, A and b) show that the corallites in the outer part of the 





^HXr 



Fig. 23. — Minute structure of 3/. siibpulchcUa, Nich., from the Cincinnati Group of Ohio. 



A, Part of a tangential section, embracing one of the maculae, enlarged eighteen times ; 



B, Small portion of the same section, enlarged fifty times ; c, Vertical section of the 

 corallum, enlarged six times, showing the different structure of the corallites in the axial 

 and peripheral portions of their course respectively ; D, Part of the preceding section, 

 enlarged eighteen times. 



corallum have thickened walls, and are apparently completely 

 amalgamated with one another. 



The average corallites are 



