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. ]
Cidaris (_Hemicidaris?_) intermedia, _Lign. 101, fig. 1, 2, 3._--The shell of this echinite, from the Oolite at Calne, so closely resembles that of _H. crenularis_, above described. that without the spines it could not be distinguished.[298] It is of a depressed form, and has very long subcylindrical spines; a specimen is figured _Lign. 102_, _fig. 9_. It is this cidarite which occurs in such immense numbers in the Oolite at Calne, Chippenham, Faringdon, &c. Slabs of limestone are occasionally extracted from the quarries at Calne, with more than thirty of these echinites surrounded by their spines. I have one specimen, attached to a block of limestone, with fifty spines; but it is difficult to detach a perfect spine.
[298] Mr. Woodward. I am indebted to this gentleman for many valuable remarks on fossil Echinoderms.
Cidaris Blumenbachii. _Lign. 127, fig. 3._--This is another beautiful characteristic Turban Echinite of the Oolite. The tubercles are very large and prominent, and the spines remarkably neat, being covered with longitudinal granulated striæ; they are of an elongated cucumerine form, and homogeneous in structure (_Lign. 127, fig. 5_). They occur by hundreds in some of the layers of friable stone in the quarries around Calne.
Many species of Turban Echinites abound in the White Chalk, especially near Gravesend, Northfleet, Purfleet, Charlton, and other places in Kent; the softness of the cretaceous strata in those localities rendering the removal of the chalk an easy task. Splendid specimens, with the spines and tubercles almost as fresh as if recent, have rewarded the patience and skill of collectors. The British Museum contains many fine examples, especially a specimen of _Cidaris clavigera_ from Charlton, with sixteen spines, and the dental apparatus _in situ_.[299]