PLATE XIII. 



Fig. I. Part of a tangential section q{ Moiiticulipora (Heterotrypa) mammulata, 

 D'Orb., from the Cincinnati Group of Cincinnati, Ohio, enlarged twenty 

 times, showing the two sets of corallites. 



Fig. I a. Small portion of the preceding section, enlarged fifty times, showing 

 the structure of the walls, and the occasional presence of spines (modified 

 corallites). 



Fig. I b. Part of a vertical section of the same species, enlarged twenty times, 

 showing the different disposition of the tabuls in the two sets of corallites 

 respectively. 



Fig. 2. Part of a tangential section of MoiitimUpora {Heterotrypa) rainosa, F. 

 and H., from the Cincinnati Group of Cincinnati, Ohio, enlarged twenty 

 times, showing'the two sets of corallites. 



Fig. 2 a. Part of a vertical section of the same species, enlarged twenty times, 

 showing the different disposition of the tabulse in the two sets of tubes. 



Fig. 3. Under surface of a .specimen oi MoiitiLiilipora{DipIoti-ypa) pdropolitana, 

 Pand., from the Lower Silurian rocks of Sweden, of the natural size, show- 

 ing the epithecal plate. 



Fig. 3 a. The same specimen, viewed in profile, of the natural size. 



Fig. 3 b. Part of a tangential section of the same, enlarged twenty times, show- 

 ing the uniformly thin walls of the corallites, and the small angular tubes 

 wedged in at the angles of junction of the larger ones. 



Fig. 3 c. Part of a vertical section of the same, showing the two sets of tubes, 

 and the increase in the number of the tabute as the exterior is approached, 

 — enlarged twenty times. 



Fig. 4. Under surface of MonticuUpora {Diplotrypa) ]]7titcavcsii, Nich., of the 

 natural size, showing the concave base and epithecal plate. From the 

 Trenton Limestone of Peterboro', Ontario. 



Fig. 4 a- The same specimen viewed in profile. 



Fig. 4 b. Part of a tangential section of the same, enlarged twenty times, show- 

 ing the different groups of corallites. 



Fig. 5. Side view of a si;ecinien of Miviticulipora {Moiwtrypa) Wiiiteri, Nich., 

 of the natural size, from the Devonian Limestone of Gerolstein in the 

 Eifel. 



Fig. 5 a. Under surface of the same, of the natural size, showing the epithecal 

 plate partially removed by weathering. 



