632 ECHINODEKMATA ArbaciadoK. Tubercles smooth, imperforate ; four large anal plates ; auriculae closed. JEchinidae. Tubercles imperforate, or perforate and crenulate ; anal plates numerous ; pairs of pores in ranks of three, four, or more. ii. Shell oval or elliptical. Ecliinoinetridae. Pores in ranks of five or six pairs. B. Anus eccentric through intervention of one or several super numerary apical plates. Salcnidcc. All fossil forms, with exception of Salenia rarispina. Group II. IEREGULAKIA or EXOCYCLICA. Anus eccentric, not within the apical disk. A. Ambulacra simple, not petaloid. Galeritidai. Mouth central ; shell globular or sub- pentagonal ; a single apex at which the ambulacra converge. Dysasteridcc. Mouth eccentric ; shell ovoid or heart- shaped ; two apices, at which the bivium and trivium respectively converge. B. Ambulacra more or less petaloid. i. Dental apparatus present. ClypeastridoE. Shell more or less flattened, sub-penta gonal. Scutcllidce. Shell depressed, discoidal, often digitate or perforated; lower surface with ramifying grooves, ii. Dental apparatus absent. Cassidulidce. Mouth central or nearly so ; peristome sub-pentagonal. Spatangidce. Mouth eccentric, transverse or reniform. To the group Regularia must be added the Cretaceous and Recent family of Echinothuridoc. The Echinidea are represented in Palteo- FIG. 8. Fossil Echinidea. 1. Palsechinus sphaericus, Scouler; Carboniferous, Ireland. 2. Archaeocidaris Uiii, Flem. (spine and intermediate plate) ; Carboni ferous, Ireland. 3. Cidaris glandifeni, Goldf. (spine) ; Jura, Moimt Carmel. 4. Hemicidaris intermedia, Flem. ; CoraJlian, Calne. 5. Salenia petalifera, Desm.: U. Greenland, Wilts. 6. Dysaster ringens, Ag ; Inferior Oolite, Dorset. 7. Hemipneustes Greenovii, Forbes; U. Greenland, Blackdown." 8. Catopygus carinatus, Goldf. ; U. Greemand, Wilts. FIG. 9. Fossil Echinidea. 1. Cidaris florigemma, with spine, a, and single ambulacra! plate, magnified, b (after Wright); Coral Rag. 2. Ananehytes ovatus, Lam. ; U. Chalk, Europe. 3. Pygastersemisulcatus, Ph.; Inf. Oolite, Cheltenham. 4. Galentes albogalerus, Lam.; U. Chalk. Kent. 5. bcutella subrotunda ; Miocene, Malta. zoic strata by the Tesselata or Perisehoechinidse, Palocchinus (see 1 m fig. 8), PenschodomUs, Lepidechinus, Eocidaris, Archasondaris (2 in fig. 8), Melonites, Oligoponis, and Lepidesthes (see Quart. Jour. Geol. Soc., xxx. 307). The Echinidcs and Dysasteridcc occur first in the Trias, and are represented by numerous species in Mesozoic strata; the Salcnidce, Gakritidce, and Cassidulidce make their earliest appearance in rocks of Jurassic, and the Spa- tangidce, including the sub-family Ananchytidce, in rocks of Cre taceous age. The accompanying figures represent fossil forms of Echinidea characteristic of various strata. Some account of the distribution in space of the Echinidea, together with that of other classes of the Echinodermata, will be found at page 278 of the present volume. See also Sir Wyville Thomson, Proc. Roy. Soc., xx. 1872. Order IT. ASTEIUDEA. The Asteridea, or Star-fishes, have mostly a star-shaped body, composed of a central disk and five or more rays. The common British species of Solaster, S. papi>osus (fig. 10), has ordinarily 13 rays; FIG. 10. Solaster papposus (upper surface). S. hdianthoides, a South American species, has as many as 34, the extinct S. Moretoni of the Great Oolite had 33, and another fossil species, of Devonian age, Helianthaster Rhenanus, had 16 rays. The rays are sometimes very short (fig. 11), or altogether wanting, the body having FIG. 11. Astrogonium phrygianum (upper surface), the form of a pentagonal disk. In the Srisingidce they may attain a length of many feet. The perisome iu the Asteridea is coriaceous, and consists of an ectoderm with a thin ciliated cuticle, a muscular mesoderm which
contains calcareous skeletal plates or ossicula, and an