196 MOLLUSCA. 



spaces. The outer surface of the branches is minutely porous 

 or longitudinally striated. The inner surface of the branches 

 exhibits a central ridge or keel, separating the mouths of two 

 rows of cells. Sometimes there is an additional row of cells 

 on the mesial keel, and the dissepiments are usually destitute 

 of cells. The entire ccenoecium is calcareous. In the nearly 

 allied genus Retepora, the coencecium is also a fan-shaped ex- 

 pansion, and is also of a calcareous consistence. In place, 

 however, of transverse dissepiments, the branches of the ccenoe- 

 cium unite with one another in such a manner as to form ovate 

 interspaces or " fenestrules." The outer surface of the ccenoe- 

 cium is non-celluliferous and minutely striated. The inner 

 surface bears several rows of small cells. 



In the genus Ptilopora (fig. 134) are forms essentially similar 



Fig. 134. Ptilopora pluma; the right-hand figure of the natural size, the left-hand 

 figure enlarged. Carboniferous. 



to Fenestella, but having a feather-like arrangement, consisting 

 of a central stem giving off lateral branches, which are con- 

 nected by dissepiments, leaving oval fenestrules. In Glauco- 

 nome (Acanthocladia, King) are forms in which the ccencecium 

 consists of a central axis giving off lateral branches, which bear 

 longitudinally-disposed cellules, but which are not united by 

 transverse dissepiments. Lastly, in Archimedipora, which is 

 found abundantly in parts of the Carboniferous series of the 

 United States, the ccenoecium is wound in an oblique spiral 

 round a central axis. The only other Palaeozoic genus of any 

 special importance is Ptilodictya, the species of which are 

 especially characteristic of the Silurian Rocks. In this genus, 

 the ccenoecium is flattened, foliaceous, or more commonly 



