ECHINOIDEA Char.—Body spheroid or discoid, incased in a crust of in flexibly-joined calcareous plates, and armed with spines ; dental system complex, arranged so as to resemble a " lantern." The Echinoitha appear first in the carboniferous limestone and attain their maximum in the cretaceous strata. In all secondary and more modern Echinidce, the shell is composed of five double rows of ambulacral plates, and five inter-ambu lacral ; but in the Palachinus (fig. 8, a) of the carboniferous limestone there are six rows of inter-ambulacral plates, and in Perischodomus five. Only detached plates of Archeeocidaris i. Pabechinus sphiericus, Scowler ; Carboniferous, Ireland. s. Arclueocidaris Urii, Flem. ; Carboniferous, Ireland.
3. Cidaris glandifera, Goldf. (spine); Jura, Mount Cannel.
(fig. 8, 4, have been seen, and these, by their six-sided form, seem also to have been arranged in more than double series. Normal Echinklie, of the existing genus Cidaris, abound in the upper trial. Some of the secondary species of Cidaris have the ambulacral pores widely separated (=Rhabdoci daris) ; in others the rows of pores are doubled (=Diploci daris). The genus Hemicidaris (fig. 8, 4), distinguished by the large spine-bearing tubercles on the lower part of the ambulacra, ranges from the trim to the chalk-marl. Diadema•, with smooth, solid spines (=Hemicliadema), appear in the lias, and continue to the chalk, where the modern type, with annulated, hollow spines, appears. Echinopsis also occurs in the has ; and Acrosalenia, a genus characteristic of the oolites, and distinguished from Salenia by its perforated tubercles. Acrocidaris and Heliocidaris, with Glypticus, and several other sub-genera of Echinus, are also peculiar to the oolites. Salenia (fig. 8, 5) with its ornamental disk, is characteristically creta ceous. Arbacia and Temnopleurus appear first in the eocene. The Cassidulidce commence in the oolites, with Pygaster (fig. 7, i) and Holectypus, and abound in the cretaceous system. Galerites (fig. 7, 3), Discoid,ea, Pyrina, and Cassidulus are pecu liar to the chalk. The Cl,ypeastridce are represented in the
oolites by numerous species of Echinolampas and Hucleolites (or Clypeus); the latter genus attains a large size. The sub genus Catopygus (fig. 8, s) is peculiar to the cretaceous series. Conoclypeus occurs in the chalk and tertiaries. Clypeaster flourished most in the miocene age ; many large species are found in the south of Europe, Madeira, and the West Indies. Numerous genera, remarkable for their flattened form, and popularly known as " cake-urchins," are peculiar to the ter tiaries and existing seas. Lenita and Scutellina are eocene ; Scutella (fig. 7, 4) is miocene. Mellita and Echinarachnius are both fossil and recent. The heart-shaped urchins (Spatan gichx), are only remotely represented in the oolites by Disaster (fig. 8, 6) ; they are numerous in the chalk, to which Micraster, Epiaster, Hentiimeustes (fig. 8, 7), Archiacia, Holaster, and ,1 nanchytes (fig. 7, z), are peculiar. Toxaster is characteristic of the lower neocomian. Hemiaster is cretaceous and tertiary. Spatangus, Eupatagus, Brissus, Amphidotus, and Schisaster are tertiary and recent forms.
The shell of the Echinodermata has the same intimate structure in all the orders and families, and in every part of the skeleton, whether " test," or " spine," or " tooth." The smallest plates resemble bits of perforated card-board, and the largest and most solid are formed of a repetition of similar laminae. In a few membranous structures, minute spicula, curved, bi-hamate, or anchor-shaped, are met with. They are always composed of carbonate of lime ; but owing to their porosity, fossil examples are commonly impregnated with earth, or pyrites, or silica, and form bad subjects for micro scopic investigation. Without, however, losing their organic structure, the fossil Echinoderms exhibit a cleavage like that of calcareous spar, by which the smallest ossicle of star-fish or Crinoid may be recognised : this peculiarity is most strikingly obvious in the great spines of the Cidaris (fig. 8, 3), or the enlarged column of the " pear Enerinite " (fig. 6, 7). Examples of the latter may be seen which had been crushed when recent, and before the sparry structure was superinduced.