PALEONTOLOGY OF IOWA. 651 



FENESTELLA ( MILLER). 

 Submenus Archimedes ( LESUEUR). 



THE Bryozoans designated as above by LESUEUR do not 

 differ in their essential structure from FENESTELLA : their 

 mode of growth, however, is quite distinct ; the flabelliform 

 expansion acquiring a solid central axis, around which it 

 revolves in an ascending spiral form, spreading equally on 

 every side*. 



Archimedes wortheni. 



PLATE XXII. FIG. 3; 4 a, b; 5 a, b. 



Fenestella (Archimedes) wortheni : HALL, Proceedings of the American Association for the 



Advancement of Science, 1856, p. 176. 



Axis strong, robust, having a little more than three volu- 

 tions in an inch (or from three to four in the same space) : 

 axis and base of expansion rugose, the latter extremely 

 thickened and often supported by oblique braces of solid 

 calcareous matter. Frond broadly spreading, ascending, ex- 

 ternally marked by oval fenestrules and strong closely 

 arranged connecting processes. Branches, on the inner or 

 celluliferous side, rounded, with a faint sharp meandering 

 ridge down the middle, which is sometimes thickened, with 

 usually a single row of cells on each side : cells round, 

 opening vertically, with the margins frequently thickened ; 

 three in the space of each fenestrule. A third row of cells 

 sometimes just below the bifurcation, and also on the con- 

 necting processes. 



This species is remarkable for its thick strong axis, and for the finely fenestrate 

 frond, which presents some variations in its characters according to age and other 

 circumstances. 



* See " Observations on the Genus ARCHIMEDES or FENESTELLA, with descriptions of 

 species, etc." : Proceedings of the American Association for the Advancement of Science, 

 1856, p. 176. 



