422 MOLLUSCA. 



branches, which are connected by dissepiments, leaving oval 

 fenestrules. 



Among the more abnormal forms of the Fenestellidce, Hemi- 

 trypa comprises Devonian and Carboniferous forms, in which 

 the coencecium is funnel-shaped and reticulate, as in Fenes- 

 tella ; but the branches have high ridges externally, and the 

 crests of these are united by cross-bars or dissepiments, which 

 are so thickened laterally as to leave only small round pores. 

 The pores open into galleries between the branches, and 

 these open internally by a second set of pores, the mouths of 



Fig. 203. Ptilopora pluma ; the right-hand figure of the natural size, the left-hand 

 figure enlarged. Carboniferous. 



the cells themselves opening into the galleries just mentioned. 

 Another remarkable genus is Archimedipora (fig. 264, c), 

 which occurs abundantly in parts of the Carboniferous series 

 of North America. In this genus the ccencecium is wound 

 in an oblique spiral round a solid central screw-like axis. 

 The fan-shaped expansions of the frond are simply reticulate 

 externally, but the branches carry a double row of cell-pores 

 internally. In the genus Glauconome (Acanthocladia, King), 

 again, the ccencecium is branched in an irregular manner, 

 and the lateral off-sets (fig. 264, 1) are not united by dissepi- 

 ments. The one side is simply longitudinally striated, but 

 the other side exhibits a double series of alternately placed 

 cell-mouths, other cells being placed on the small side-shoots 

 of the frond. The genus is Carboniferous and Devonian. 



