Cystoid

1. Triamara cutleri/ventricosa
2. Holocystites scutallatus
3. Holocystites sp.

• Silurian
• Massie (formerly Osgood) Shale
• Napoleon, Ripley County, Indiana, USA

Size: 7 cm theca for the Triamara

This is an impressive slab of six naturally-associated diploporite cystoids: one Triamara and five Holocystites. The Triamara has been prepped so that it can "pop out" of its original pocket.

1. Here is one of my non-crinoid echinoderms, a large cystoid known as Triamara cutleri (a.k.a. Triamara ventricosa). It's from the Silurian Massie/Osgood Shale of Indiana, which produces an unusual abundance of these diploporite cystoids relative to other echinoderms. Triamara is a cystoid in the shape of an elongated, bulbous sac made of porous plates. Its most interesting and unique feature is the three unequally-long ambulacral grooves at the top of the theca which join to form a characteristic "Y" shape (also note the anal opening to the left of the "Y"). At the end of each ambulacral groove are the attachment sites for a pair of brachioles, indicating the cystoid posessed a total of six filter-feeding arms.