632 



MOLLUSCOIDEA. 





cells (fig. 476, D and f). The cell-mouths are trilobed, and the surface 

 in well-preserved specimens exhibits shallow interapertural pits (fig. 476, 

 e), which in thin sections are seen to be produced by irregular vesicles 

 occupying the intervals between the proper zocecia (fig. 476, c). This 

 species is abundant in the Devonian rocks of North America, but is 

 placed by Hall in the genus Stictopora. The genus Coscinium of Key- 

 serling ( = Coscinotrypa, Hall) possesses an internal structure essentially 

 similar to that of Cystodictya, but the polyzoary forms a wide netted 

 expansion (fig. 477), perforated by numerous oval " fenestrates," the 

 edges of which are sharp, and are non-celluliferous. In this genus the 

 interstitial vesicular tissue between the zocecia is largely obliterated by 

 a finely tubulated calcareous deposit (fig. 455, c). 



The family of the Ceramoporidcz has been founded by Mr Ulrich 

 for a number of Palaeozoic Polyzoa in which the polyzoary is usually 

 incrusting, the cell-apertures being triangular or ovate, generally with 

 a prominent and arched lip on one side. The type-genus of this 

 family is Ceramopora itself, which ranges from the Ordovician to the 

 Devonian, and forms thin crusts upon corals, shells, &c. The cells 



Fig. 478. — Fragment of 

 Rhovibopora Hamilton- 

 ensis, of the natural size 

 and enlarged. Devonian, 

 Canada. (Original.) 



i 





Fig. 479. — Rhabdomeson gracile, from the Car- 

 boniferous rocks of Scotland, a, Tangential section, 

 enlarged, b, Vertical section, enlarged, showing the 

 central tube : c, The proper cell-mouth ; v, The 

 outer chamber or "vestibule." (Original.) 



in this genus are angular, with a strongly arched lip and oblique 

 aperture, and they usually radiate from one or more centres of 

 growth. 



Finally, the family of the Rhabdomesontidce includes a number of 

 small Palaeozoic Polyzoa, in which the polyzoary is ramose, and is 

 composed of slender, cylindrical, solid or tubular branches, the cell- 

 apertures being placed on all sides of the branches (fig. 478). The 

 proper cell-mouth is placed at a little distance below the surface, 

 and opens into a so-called " vestibule " or outer chamber, which 

 constitutes the apparent cell-aperture on the surface (fig. 479, b). 

 The cavities of the zocecia may be crossed by a limited number 

 of "tabulae," and the apertures are sometimes provided with per- 

 forated " closing-plates " ; while thick-walled tubes (for the support 



