BRYOZOA. 565 



near the base. Here the dissepiments are thickened so as to 

 equal the branches and fenestrules in width Toward the outer 

 margin both the branches and dissepiments gradually become 

 thinner, and the fenestrules wider, and oval or even sub-quad- 

 rate in form. The surface of the branches is rounded and, when 

 well preserved, minutely granular. Between ten and eleven 

 branches occur in 5 mm., and in the same space longitudinally 

 nine or ten fenestrules. 



The form of the zoarium, carinated bars of the superficial 

 net-work, small interstices, and rounded branches separate this 

 species from H. proutana. H. perstriata has very narrow fenes- 

 trules and strongly striated branches. 



Position and locality: Keokuk group, Keokuk, Iowa. 



ARCHIMEDES LeSueur (Owen), 1842. 



(Amer. Jour, of Sci. Vol. 43, p. 19.) 

 (For generic diagnosis, see page 396.) 



This remarkable genus of the FENESTELLID.*} is so far known 

 only from American Lower Carboniferous deposits. Here, how- 

 ever, the genus suddenly springs into prominence, first making 

 its appearance with four species in the Keokuk group, (includ- 

 ing the Warsaw beds*), two of them, the A. grandis Ulr., and 

 A. wortheni Hall, being the largest yet discovered. In the St. 

 Louis limestone, in which the Fenestellids are few and but illy 

 preserved, no species of Archimedes has yet been met with, but 

 in the overlying Chester group, the individuals and species again 

 become numerous, being, perhaps, the most characteristic fossils 

 of that division. Much diversity of opinion exists as to the 

 possibility of separating the species of the genus by the charac- 

 ters of the axis, but no one, so far as I am aware, has hereto- 

 fore given them much study, nor does it appear that any one 

 has taken pains to collect much material, and I do not wonder 

 then that so many palaeontologists have expressed themselves 



* These beds are usually classified with the St. Louis group, but judging mainly 

 from the Bryoza, I should regard them as more intimately connected with the Keokuk 

 group. That they are beds of passage between these two groups will scarcely admit of 

 doubt. 



