Next are a couple of specimens from my collection.Note the ridge along the top of the first two pictures. The stem attachment point is on the middle right in the first picture (just below my thumb).
A view of the stem attachment point.
A second specimen that is a little more crushed.
There are several species of Platycystites listed as being known from the Bromide formation. The only good pictures I can find online are here from the Iowa Digital Library. Based on their pics I'm inclined to label these specimens as Platycystites infundus.
The Bromide formation is upper Ordovician in age (Blackriverian [N. American stages] or Sandbian [ICS stages]). The above specimens came from the Mountain Lake Member of the Bromide formation and were found in the 1970's near Davis, Oklahoma.