=Characters of Sea-urchins.--= Echinoids are animals enclosed in a spheroidal box or test composed of numerous calcareous plates, disposed geometrically as in the Starfishes, along five principal lines. The test in the living condition is more or less densely covered with spines. The mouth is on the under surface. The anus is either on the top of the test (dorso-central), or somewhere in the median line between the two lower ambulacra. The ambulacra ("a garden path") are the rows of perforated plates on the upper (abactinal) surface sometimes extending to the lower surface, through which protrude the podia, which in Starfishes are situated in grooves on the lower surface. =Silurian Palaeechinoids.--= The Palaeechinoids are represented in the Silurian of Australia by occasional plates, as at Bowning, New South Wales, and near Kilmore, Victoria, whilst spines are not uncommon in certain Silurian limestones at Tyer's River, Gippsland. =Carbopermian Palaeechinoids.--= In the Carbopermian of New South Wales, tests of _Archaeocidaris_ have been recorded, and also a plate of the same genus in the Gympie Beds of Rockhampton, Queensland. =Regular Echinoids.--= The regular Echinoids date from Permian times. They have two vertical rows of plates for each ambulacrum and inter-ambulacrum. The mouth is on the underside, and the anus abactinal (on the upper side) and near the centre. [Illustration: =Fig. 80.--CAINOZOIC SEA-URCHINS.= A--Cidaris (Leiocidaris) australiae, Duncan sp. Cainozoic (Janjukian). Cape Otway, Victoria B--Psammechinus woodsi, Laube. Cainozoic (Janjukian). Murray River Cliffs, S. Australia C--Fibularia gregata, Tate. Cainozoic (Janjukian). Aldinga, S.A. D--Echinocyamus (Scutellina) patella, Tate sp. Cainozoic (Janjukian). Torquay, Victoria E--Clypeaster gippslandicus, McCoy. Cainozoic (Janjukian). Bairnsdale, Victoria F--Studeria elegans, Laube. sp. Cainozoic (Janjukian). Murray River Cliffs, S. Australia. ] =Cainozoic Regular Echinoids.--= In Australasia they make their first appearance in strata of Tertiary age, and some species, as _Paradoxechinus novus_, range through Balcombian strata to Kalimnan in Victoria, or Oligocene to Lower Pliocene, but are more typically Janjukian. _Echinus_ (_Psammechinus_) _woodsi_ (Fig. 80 B) is common in Janjukian strata in Victoria and South Australia and occurs sparingly in the Kalimnan. Another common form of the regular Echinoids in Southern Australia is _Cidaris australiae_ (Fig. 80 A), ranging from Janjukian to Kalimnan, occurring more frequently in the older series. In New Zealand a species of _Cidaris_ (_C. striata_), is known from the Oamaru Series at Brighton. An _Echinus_ occurs in the Oamaru Series of Broken River, and two species of that genus in the Wanganui formation of Shakespeare Cliff. _Temnechinus macleayana_ has been recorded from the Cainozoic (Miocene or Pliocene) of Yule Island, Papua. =Irregular Echinoids.--=