Orophocrinus stelliformis scale in cms CLASS - Blastoidea ORDER - Fissiculata FAMILY - Orophocrinidae GENUS/SPECIES - Orophocrinus stelliformis ROCK UNIT - Burlington Limestone AGE - early Mississippian (350 million years old) LOCALITY - Hannibal, Marion Co., Missouri, USA Blastoids are a "conservative" group of crinoid-like echinoderms. Although they were common throughout much of the Paleozoic Era, they changed very little through time. Compare the theca (main body portion) of Orophocrinus (above) with the theca of Pentremites. The thecal plates differ in shape (making recognizable species), but the number and position of these plates remains remarkably consistent through their entire history. PRI 42165