Genera of the Xorth American Palaeozoic Bryozoa. 519 



Archimedes, Le Sueur. 



(Amer. Jour. Sci., Yol. 43, p. 19. 1842.) 



Type, Archimedes Owenana, Hall (sp.). 



The forms in this genus do not differ in their essential struc- 

 ture from Fenestella ; their mode of growth, however, is quite 

 distinct; the expansion acquiriog a solid central axis, around 

 which it revolves in an ascending spiral form, spreading equally 

 on every side. On the celluliferous face, the cell apertures are 

 arranged in two parallel rows, separated by a carina or row of 

 nodes. 



Fig. 71. Archimedes Wortheni, frond natural size. 

 Fig. 72. A longitudinal section of tlie central axis. 

 Lower fig. A transverse section of the central axis, x2. 



Ptilopora, McCoy. 

 (Syn. Carb. Foss. Ireland, p. 200.) 



Type, Ptilopora flustriformis^ McCoy. 

 (Plate 8, figs. 1-7.) 



Flabelliform, attached by roots, from which a strong midrib 

 arises, giving origin on each side to thin equidistant branches, 

 which are connected by regularly disposed dissepiments ; cellu- 

 liferous on one face only ; cell apertures disposed in two longi- 

 tudinal, parallel rows, separated by a carina or row of nodes. 



