POLYZOA OR BRYOZOA. 421 



small interspaces. The outer surface of the branches is 

 minutely porous or longitudinally striated. The inner sur- 

 face of the branches exhibits a central ridge or keel, separat- 

 ing the mouths of two rows of cells. Sometimes there is 

 an additional row of cells on the mesial keel, and the dis- 

 sepiments are usually destitute of cells. The entire ccenoe- 

 cium is calcareous. In the nearly allied genus Phyllopora or 

 Retepora, 1 the ccenoecium is also a fan-shaped expansion, and 

 is also of a calcareous consistence. In place, however, of 

 transverse dissepiments, the branches of the coencecium unite 

 with one another in such a manner as to form ovate inter- 

 spaces or " fenestrules." The outer surface of the ccencecium 

 is non-celluliferous and minutely striated. The inner surface 

 bears two or more rows of small cells. The genus ranges 

 from the Lower Silurian to the Permian. Actinostoma, of 

 the Carboniferous, is like Fenestella in having a fenestrated 

 frond, poriferous on one side only, but the branches are not 

 keeled, the cell-mouths are furnished with radiating teeth, 

 and the cells have a second smaller aperture close to their 

 ends. Polypora (fig. 264, a), abundant in the Devonian and 

 Carboniferous, resembles Fenestella in general form ; but the 

 branches are not keeled, and are pierced by from three to 

 five rows of cell-openings, while the " dissepiments " are solid 

 and non-poriferous. Synocladia, of the Carboniferous and 

 Permian, has the funnel - shaped and reticulated frond of 

 Fenestella and Polypora, but the dissepiments are directed 

 obliquely upwards, bearing two rows of pores each, while the 

 branches carry from three to five rows of pores separated by 

 keels. Carinopora, again, of the Devonian, has the external 

 features of Retepora ; but the branches are furnished inter- 

 nally with immense keels, bordered on each side by a single 

 row of pores. Lastly, in Ptilopora (fig. 263) are forms 

 essentially similar to Fenestella, but having a feather -like 

 arrangement, consisting of a central stem giving off lateral 



1 The name Retepora has become so firmly attached to this genus, that it 

 appears difficult to dislodge it now. The Palaeozoic types so named are, how- 

 ever, altogether without relationship with the more modern true Reteporos, 

 and it would therefore be best to adopt for them Prof. King's name of Phyl- 

 lopora. 



