Rostroconchs: Paleozoic taco shells

The Paleozoic was full of invertebrate groups that didn't quite hack it. The kings of extinct invertebrates are, of course, trilobites. The next tier down, widely known to paleontologists, geologists, and fossil collectors, is where we find things like tabulate corals, rugose corals, and eurypterids ("sea scorpions"). Then there are some that linger in obscurity; the aglaspids (Cambrian critters that were something like trilobites with poor-quality horseshoe crab disguises), for example, or cyclocystoids (disc-like echinoderms), which of this writing don't even have a Wikipedia article. Rostroconchs are another group that hangs out near the aglaspid/cyclocystoid end of the scale.