Gastropods, or snails, had a single long tubed shell coiled into a spiral. The name means “stomach foot”, since they possessed a muscular foot which they used to glide along the sea bottom.
Although they look like Ammonites, the latter is a cephalopod (“Head Foot”) which is more evolved and more like modern squids and octopuses.
More information on gastropods may be found here: http://paleo.cortland.edu/tutorial/Ceph%26Gast/gastropods.htm
Ancient Seas illustration and creature cutouts ©Emily S. Damstra