424 



MOLLUSCA. 



cate, striated, calcareous membrane (fig. 265), and opening 

 on the surface by round or oval mouths, while the margins 



of the frond are striated longi- 

 tudinally and destitute of cell- 

 mouths. The Heterodidya of 

 the Devonian resembles a large 

 Ptilodictya in form, and con- 

 sists similarly of two layers of 

 cells, separated by a thin cen- 

 tral lamina, and opening on 

 opposite sides of the flattened 

 frond ; but there is the differ- 

 ence that the cells are divided by 

 transverse partitions or " tabulae," 



. 



Structures which are Other wise 

 , , ,-, 



almost unknown amongst the 

 forms universally recognised as 



Polyzoa. Clathropora, again, of the Upper Silurian and De- 

 vonian, quite resembles Ptilodictya in internal structure ; but 

 the ccencecium (fig. 266) is in the form of a broad fan-like 

 expansion, perforated by regularly - placed round openings, 

 the margins of which are striated and non-celluliferous. 



^.-ptuodictya faidformis. a, 

 smaii specimen of the natural size ; b, 



Cross-section, showing the shape of the 

 frond; c, Portion of the surface, enlarged. 

 Trenton Limestone and Cincinnati Group, 



America. (Original.) 



Fig. 267. Fragment of 

 Ceriopora Hamiltonensis, of 

 the natural size and en- 

 Fig. 26t3. Fragment of Clathropora intertexta, of the natural larged. Devonian, Canada, 

 size and enlarged. Devonian, Canada. (Original.) (Original.) 



As the representatives of another great group of Palae- 

 ozoic Polyzoa, we may take the Silurian, Devonian, and 

 Carboniferous forms which have been described under the 

 names of Ceriopora, Ehabdomeson, Trematopora, and Helopora. 

 All these have the form of slender rounded stems, usu- 



