Important Phanerozoic reef type generally formed in deeper water on carbonate shelf-to-basin slopes. They contain radiaxial calcite spar-filled depositional cavities (stromatactis) but lack abundant metazoan framebuilders. Mud-mounds sometimes attained widths and depositional reliefs of over a hundred meters. Stromatactoid mud-mounds are a framework of submarine-cemented, crudely reticulate masses or a succession of laminar crusts, surrounded by cement-filled cavities. Cavities developed when sediment around the framework was removed by winnowing. Mud-mound frameworks show strong similarities to shallow-water cryptalgal structures, especially thrombolites which occur in many Phanerozoic shallow-water reefs. Both thrombolites and mud-mound frameworks contain fenestral fabric and diagnostic cryptalgal microstructure. Their depositional surfaces were irregular, not bored or burrowed, and rarely encrusted. Stromatolitic lamination occasionally occurs in mud-mounds. It is proposed that mud-mounds accumulated from organic (probably blue-green algal) binding of locally derived lime mud and bioclasts deposited relatively slowly from suspension. Sporadic rapid sedimentation temporarily smothered the living algal mat and left layers of unbound sediment. Periods of winnowing washed out unconsolidated sediment and promoted synsedimentary cementation of resulting stromatoctoid cavities and algal-bound framework.--Modified journal abstract.