GENERA OF FAVOSITIDJE. 



95 



liquely on the surface. Walls thickened, and completely amal- 

 gamated in contiguous tubes. Calices subtriangular, oval, semi- 

 lunar, or sometimes fissure-like, with thickened margins, but 

 generally with a sharp lower lip which may be indented with 

 one or more emarginations. The arrangement of the calices 

 is very irregular, but they are often disposed in diagonal lines 

 running across the frond, and there are usually five or six of 

 them in the space of two lines. One or two inward projections 

 of the wall of the corallites on one side may represent septa ; 

 or these structures may be wholly wanting. Tabulae obsolete, 

 or few, remote, and complete. Mural pores apparently very 

 few and remote. 



Obs. In its general habit and form this species (fig. 17) 

 closely resembles P. Fischeri, Bill., both constituting thin flat- 



rig. 17. A, A fragment of Pachypora frondosa, Nich., of the natural size, shown as if 

 attached to some foreign body, and with the extremities of the frond restored in outline ; 

 B, A small portion of the surface of the same, enlarged five times ; c, Tangential section 

 of the same, enlarged seven times ; D, Vertical section of the same, similarly enlarged ; 

 E, Section at right angles to the flat surfaces of the frond, enlarged twice, showing the 

 divergence of the corallites from a central plane, together with a few tabulse. From the 

 Hamilton group of Arkona. 



tened fronds, attached basally to some foreign object, and hav- 

 ing the corallites so arranged in reference to the median plane 

 of the expansion as to open over the whole of both of the flat 



