Plasiacystis mobilis was named by Prokop and Petr (2004) [= 1887 Echinosphaerites infaustus BARRANDE; Barrande, Pl. 25, fig. 15 (specimen with anterior-posteriorly strongly compressed theca, and also with a proxistele preserved); Pl. 39, fig. 29. (Fig. 30 shows a specimen that represents probably true “Echinosphaerites” with theca composed of large plates bearing both tubercules as well as short transverse channels on their borders) = 1918 Mitrocystella Barrandei JAEKEL; Jaekel, p. 121, fig. 110 H, J, K, L (rigid paddle-like dististele belonging probably to two specimens or even more) = 1947 Lepidocoleus bohemicus sp. n., PRANTL MS (proxistele)]. Its type specimen is NM L 13216 and is a 3D body fossil. Its type locality is Osek, which is in a Sarka offshore shale in the Sarka Formation of Czechia. It is the type species of Plasiacystis.