Acidiscus

Acidiscus Rasetti, 1966, is a genus of Eodiscinid trilobite belonging to the family Weymouthiidae Kobayashi T. (1943), Order Agnostida Salter (1864). It lived during the Botomian stage = late Lower Cambrian Stage 4 (upper of two stages subdividing the un-named Series 2); the upper Botomian boundary corresponds to base of the Middle Cambrian, Miaolingian Series and Wuliuan stage.
Taxonomy.
A cladogram showing the relationship between several species of the genera "Acidiscus", "Bolboparia" and "Stigmadiscus" "Acidiscus" is most closely related to "Bolboparia" and slightly more distantly to "Stigmadiscus". Synapomorphies for "Acidiscus-Bolboparia" include anterolateral cephalic spines and a large preoccipital glabellar tubercle or spine. Both of these features have been considered as being diagnostic of "Acidiscus" (e.g., Rushton, 1966; Rasetti, 1966;) but they are shared with "Bolboparia". Synapomorphies of "Bolboparia" include a sharply tapered and pointed glabella, no occipital spine, tubercles confined to posterior part of the cephalic border, strongly inflated genae, and a sculpture of tightly packed coarse granules. "Stigmadiscus" is characterised by an inflated posterior part of the glabella, the loss of anterolateral cephalic spines and a preoccipital glabellar node, and in having a very narrow cephalic border. 
Among genera currently assigned to Weymouthiidae Kobayashi, 1943 (Jell "in" Whittington "et al". 1997), Rushton (1966, p. 12) noted that, “"Acidiscus" Rasetti differs from "Serrodiscus" but slightly”, although several of the characteristics in the former genus that he listed, include the preoccipital glabellar and anterolateral cephalic spines, and the configuration of the lateral glabellar furrows.
Ecology.
"A. birdi" occurs in association with other Weymouthiidae ("Acimetopus", "Analox", "Bathydiscus", "Bolboparia", "Leptochilodiscus", "Serrodiscus"), "Calodiscus", 
several species of "Olenellus", and "Bonnia" (Dorypygidae).
Description.
Like all Weymouthiidae, "Acidiscus" lacks eyes and facial sutures. The cephalic border carries one or two pairs of marginal spines. The glabella does not reach the border furrow and has two short pit-like pairs of lateral furrows. Occipital and genal spines are present. Thorax unknown, but all weymouthiids in which it is known have three segments. The pygidium is much like that in "Serrodiscus", with an axis composed of ten rings. Pleural fields unfurrowed.