Echinus.--The shells of the genus Echinus resemble those of Cidaris in their general structure, but the tubercles are imperforate. More than twenty fossil species are described, from the Oolite and Chalk. Salenia.--In the greensand pits near Faringdon, in Berkshire, which abound in fossil sponges and other poriferæ (_ante_, p. 228), there are immense numbers of a small elegant Turban Echinite, which belongs to the genus thus named by the eminent zoologist. Dr. J. E. Gray, of the British Museum. The collector will easily recognize these sea-urchins by the plated summit. The shell has five ovarian and five interovarian plates, and an eleventh or odd one. The tubercles are crenulated. The common species at Faringdon is _S. petalifera_, of Desmarest. Two species of this genus, viz. _S. scutigera_ and _S. stellulata_, from near Warminster, are figured in _Pict. Atlas_, pl. liii. _fig._ 12, 13. _Cidarites of New Zealand._--Detached plates and spines of sea-urchins, belonging to the family Cidaritidæ, have been discovered by Mr. Walter Mantell, in the Ototara limestone of New Zealand; which is a fawn-coloured stone, composed of foraminiferæ, like the Chalk, and containing terebratulæ, corals, and teeth of sharks.[300] [300] Geol. Journal, vol. vi. p. 319. [Sidenote: SPINES OF CIDARITES.] Spines of Cidarites. _Lign. 102._--Allusion has been made to the immense numbers of the spines of two or three kinds of Cidarites that occur in the oolitic limestones of certain localities. The spines of other species and genera abound in the Chalk, Greensand, &c.; occurring detached and intermingled with corals, shells, and the usual fossils of those deposits. There is great variety in the form, size, and sculpture of these organs. In the subjoined _Lign. 102_, a few distinct kinds are represented. [Illustration: Lign. 102. Fossil Spikes of Cidarites. Fig. 1.--Spine of Acrocidaris nobilis (_Agassiz_). 2.-- ---- Cidaris cucumifera. 3.-- ---- ----- stemmacantha. 4.-- ---- ----- meandrina. 5.-- ---- ----- spinosa. 6.-- ---- ----- clunifera. 7.-- ---- ----- sceptrifera. 8.-- ---- ----- Parkinsoni. 9.-- ---- Hemicidaris intermedia. ] Flint Casts of Turban Echinites. _Lign. 103._--The siliceous casts of the shells of the Turban Echinites are interesting objects, for they are often beautiful models of the interior. A specimen of this kind is represented in _Lign. 101, fig. 5_. Casts of the larger Cidarites are often seen on the ploughed lands of the South Downs, in beds of gravel, and among the shingle on the sea-shore of chalk districts; appearing as flattened spherical bodies, with a circular protuberance at each pole, and vertical rows of nodular projections. Impressions of the external surface of the cases are also frequent on chalk-flints, and exhibit exquisite casts, in intaglio, of the mamillated tubercles, and ambulacral grooves and pores.