Cryptostome bryozoans from the standard Chazyan strata represent some of the oldest known Bryozoa. Thin-section study of the well-preserved colonies of these bryozoan species reveals that the cryptostomes had evolved into four morphologic groups, the phylloporinids, stictoporids, pachydictyids, and escharoporids, each of which was phylogenetically distinct by Chazyan time. The oldest known cryptostomes are the phylloporinids and escharoporids represented by “Phylloporina” sp. A and Chazydictya chazyensis, respectively. These species and the stictoporid, Stictopora fenestrata Hall, range through most of the 470 feet of the type Chazyan beds, which are composed of biosparite, intrasparrudite, intrasparite, and dolomite and suggest deposition in high wave-energy environments. The oldest known cryptostomes of the pachydictyid group in Chazyan strata, Pachydictya sheldonensis n. sp. and Eopachydictya gregaria n. gen. and n. sp., and the escharoporid group, Chazydictya chazyensis n. gen. and n. sp., occur abundantly in bioherms in the Sheldon Lane and Isle La Motte sections in the upper 200 feet of Chazyan strata. Eopachydictya has some features that suggest similarity to the stictoporids; however, the granulose laminate microstructure of the zooecial walls and the rudimentary tabulate interspaces indicate that its true affinities are with the pachydictyids. This genus may represent a primitive position in the evolutionary divergence of the pachydictyids from the stictoporids.