Some flabelligerid bodies are modified into a thick thorax and a long, thinner abdomen, or cauda; the thorax usually has some sediment particles cemented forming a dorsal shield, and these flabelligerids have been found boring into consolidated sediments or calcareous substrates. Two species, Pherusa inflata (Treadwell, 1914) and P. parmata (Grube, 1877), are frequently recorded from many different environmental conditions and localities. The study of all type or non-type materials of similar flabelligerids had several results: 1) The species with bodies modified into thorax and cauda, usually carrying a dorsal shield, are removed from Pherusa Oken, 1807; 2) Two body patterns are recognized on the basis of the type of neurochaetae in transitional chaetigers (3-5), and on the development of the caudae; 3) Semiodera Chamberlin, 1919 is redefined to include species with a dorsal shield variously developed, pseudocompound transitional chaetae, and caudae usually cylindrical with few neurohooks; 4) Daylithos n. gen., includes species with well-developed dorsal shield, multiarticulate or aristate transitional neurochaetae, and caudae usually depressed with few to many neurohooks; 5) Semiodera includes Semiodera caribea (Grube & Ørsted in Grube, 1859) new spelling, n. comb. (type species), S. blakei n. sp., S. curviseta (Caullery, 1944) n. comb., S. dubia (Treadwell, 1929) n. comb., S. glynni n. sp., S. inflata (Treadwell, 1914) n. comb., S. laevis (Stimpson, 1856) n. comb., S. mezianei n. sp., S. nishii n. sp., S. salazarae n. sp., S. tenera (Grube, 1868) n. comb., S. tovarae n. sp., S. treadwelli n. sp. and S. villalobosi n. sp.; 6), Daylithos n. gen. includes Daylithos parmatus Grube, 1878 n. comb. (type species), D. amorae n. sp., D. cinctus (Haswell, 1892) n. comb., D. dieteri n. sp., D. iris (Michaelsen, 1892) n. comb., and D. nudus (Caullery, 1944) n. comb.