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Solitary zooids elongate, with slightly convex anterior and a squarely cut off posterior. Test entirely smooth, moderately thick to thin. Muscles (M) 1-3 strongly fused over a wide area in the mid-dorsal region, M8 and M9 are likewise strongly fused. M1-M9 with 136-296 fibres, except for M4 with 14-40 fibres. Number of muscle fibers is due to clinal variation, in the NE Atlantic ranging from 30 to 40 in M4 and from 206 to 281 in M1-M9. Dorsal tubercule is small and C-shaped. Nucleus relatively small, compact, not causing a swelling of the test. Rudimentary elaioblast disappears in a young stage.
Aggregate zooids: body barrel shaped and typically fusiform, with conical anterior and posterior protuberances of the test that are typically long in relation to the body, giving it a slight asymmetry. Test entirely smooth, moderately thick to thin. Six body muscles; M1-M4 strongly fused over a wide area in mid-dorsal region; M4-M5 laterally fused; M5-M6 strongly fused in the mid-dorsal region. Fibre number of M1-M6 is 25-68 and is due to clinal variation. Dorsal tubercule is a simple slightly arched stick. Nucleus pale green in live specimens, relatively small and oval, not causing a swelling of the test.