The ultrastructure of some Ordovician graptoloid prosiculae

The graptoloid prosicula (conus and cauda) forms the first part of the growing colony, and was probably secreted during a single continuous episode of growth. The wall is composed of a central unit of criss-cross fibrils (/usellar Jabrlc), interlaced with parallel fibrils whose grouping appears as the spiral line (spiral line J:abric). The conus is lined with a granular or. felted sheet (sheetjfabrac). The diaphragm, at the apex of the conus, appears to be formed of fusellar fabric. The cauda is formed of fusellar and spiral fabrics with a central cavity apparently not open to tlre conus interior. An outer layer {diamond, meshJabnc) of fibrils forming a diamond mesh covers both conus and cauda, and is itself overlain by longitudi-nal rods. The apex of the cauda is formed of a second domed diaphragm of criss-cross fibrils, covered by a sheet fabric. The nema is extended distally as a series of finger-shaped increments, overlain by bandages. An outermost layer of clustered fibrils (stellafe Jabricl is occasionally present, lying on, or formed by a modification of, ttre diamond mesh fibrils of the conus