The case of the Turban Echinites is composed of twenty vertical series of plates, the ambulacra, or porous grooves, forming continuous bands from the summit to the mouth, which is armed with five angular teeth. This tribe comprises many of the most elegant fossil species; those which, from their shape and highly ornamented surface, have received the popular name of Fairy's night-caps. The genus _Cidaris_, which is characterized by perforated spinous tubercles, affords the most beautiful examples, and these are occasionally found with the spines in contact; a circumstance less rare than might be supposed, when the nature of the attachment of these organs is considered; for, upon the decomposition of the integument, and the ligaments which connect the spines with the tubercles in a living state, these appendages quickly fall off, even in recent specimens. The interesting fossil figured _Lign. 100_ (_ante_, p. 311), is a choice example of a Cidarite with the spines attached. This species (_Hemicidaris crenularis_, _Agassiz_) is said to be characteristic of the Upper Jura limestone of Switzerland, and was supposed to be identical with Mr. Parkinson's _Cidaris parpillata var._ (_Pict. Atlas_, pl. lvi. _fig._ 6), from Calne, in Wiltshire; but spines like those of _Lign. 100_, do not occur in the English oolite. These spines are not homogeneous throughout; the central part appears to have been of a less dense tissue than the outer coat, as is shown in the fractured spine in the figure. This structure does not exist in the spines of the depressed Turban Echinites, but is stated by M. Agassiz to prevail in all the species of the genus _Hemicidaris_, of which the fossil figured in _Lign. 100_ is the type. [Illustration: Lign. 101. Cidarites from the Oolite and Chalk. Fig. 1.--Hemicidaris intermedia; view in profile, showing the teeth projecting: 1/3 _nat._ _Calne._ 2.--The same seen from above, displaying the outlet, and surrounding pores. 3.--The same, view of the base, displaying the mouth, surrounded by five angular bipartite teeth. The ambulacral tubercles at the base are omitted in the figure. 4.--Diadema rotulare (_Agass._): viewed in profile: not. 5.--A siliceous cast of another species of _Diadema_. _South Downs._ 6.--View of the base of Diadema rotulare. This species occurs in the so-called Neocomian Formation of France. ]