CYCLOSTOMATA. 



631 



placed back to back, and usually more or less branched, the edges 

 of the frond being non-celluliferous, and the cell-mouths being oval 

 or circular. 



The family of the Cystodictyonida is defined by Ulrich as com- 

 prising forms of Palaeozoic Polyzoa in which the polyzoary consists 



Fig. 476. — Cystodictya Gilberti, from the Devonian rocks of North America, a, Part of the 

 polyzoary, enlarged (after Hall) ; b, Tangential section taken just below the surface, showing 

 the trilobed apertures of the cells ; c, Tangential section taken at a deeper level than the preced- 

 ing, showing the interstitial vesicular tissue ; d, Part of a specimen split along the median plane, 

 enlarged ; e, Part of the surface, enlarged, showing the porous intercellular tissue ; f, Cross- 

 section of the polyzoary, enlarged. (Original.) 



of two layers of cells placed back to back, with tubular zocecia, 

 which are separated by irregular vesicular tissue. The cell-mouths 

 have a small tooth -like 

 projection on each side, 

 giving the aperture a char- 

 acteristic trilobed form 

 (fig. 476, b) ; and the 

 margins of the zoarium 

 are sharp or rounded, and 

 are non-poriferous. 



As an example of this 

 family may be taken the 

 Devonian Cystodictya Gil- 

 berti (fig. 476), in ^vhich 

 the polyzoary is irregularly 

 branched, and consists of 



two layers of cells which open on its opposite surfaces, and are united 

 by a striated mesothecal layer formed by the union of the bases of the 



Fig. 477. — Fragment of Coscinium {Coscinotrypa) 

 cribriforme, of the natural size and enlarged. Devonian, 

 Canada. (Original.) 



