DIATOMACK.K OF NORTH AMERICA. 



Disc very convex, and strongly cellulose, without 



marginal teeth or spines Dictyopyotis cvui. 



Valves not as above 18 



f Disc without rays ; frequently hyaline ; and with 



scattered spines XANTHIOPYXIS cix. 



j All others, without strong linear rays, or large spines 



I or teeth COSCINODISCUS ox. 



