In this paper suggestions are made as to the affinities and derivation of the cryptoslomatous sub-Order Feneslelloidea. Diagnostic features of (he group are held to be the presence of zooecial apertures on the obverse of a colony only and 'longitudinal striae' on the reverse; also the presence of a primary axial complex within branches. Longitudinal striae are recognized as a fundamental and significant part of the skeletal structure. In Pseudohornera it can be seen that the striae are vestiges of formerly existing interzooecial walls which are homologous with range partitions in the Plilodictyoidea. If longitudinal slriae are vestigeal inlerzooecial walls then Iheir presence on the reverse of fencstelloid fronds suggests that this group was originally bifoliate. If. in bifoliate ancestral forms, zooecia grew back-to-back against a medial lamina, this must be represented in fenestelloid branches by the flattened primary axial skeleton on which zooecial bases rest.Fenestelloid skeletal rods are structurally identical with acanthopores in the Trepostomata and in view of their size musl be considered micracanthoporcs. The carina of biserial fenestellids represents a preferentially developed intcrzooecial wall, and carinal nodes are megacanlhopores of outstanding stature. The latter structures must have exercised a protective function but it is probable that micracanlhopores were concerned with the stabilization of the outer, soft colonial layers.There is a strong body of evidence linking the Fcnestelloidea with the Ptilodictyoidea and a tomplele morphological series bridging the gap between these sub-Orders can be assembled without difficulty. The structure of the medial lamina in Pseudoltornera is closely linked on the one hand with the primary branch skeleton of the Fenestellidae and on the other with the mesotheca of the Ptilodictyoidea. Other lines of evidence provide further reasons for believing that the fenestelloids were derived from the ptilodiclyoids, with Ihc phylloporinids representing an intermediate stock.