Our limits will not allow of a more extended notice of the fossil Echinidæ. The student should consult the Memoirs on the genera, now in course of publication at the Government School of Mines, by Professor Edward Forbes; the plates are exquisite, as works of art, and the descriptions all that can be desired.
Mr. Dixon's work contains three excellent plates of cretaceous Echinites. Several chalk species are figured in my Foss. South Down. The numerous coloured figures of fossil sea-urchins in the Pictorial Atlas of Organic Remains, have already been mentioned.
Geological Distribution of Echinites.—No vestiges of this order of radiata have been discovered in the Silurian deposits: the earliest known occurrence of any type is in the Carboniferous formation. The most ancient Echinidæ, according to the present state of our knowledge, are the Cidares, in the modified form previously noticed,—Archæocidaridæ (ante, [p. 322].).
In the Trias another type appears, Hemicidaris, which holds an intermediate place between the Cidarites properly so called and the Diadema.
In the Oolite, and Jurassic formations, numerous forms are for the first time met with, constituting the genera Echinus, Clypeus, Disaster, Holectypus, Diadema, Nucleolites, &c.
The Cretaceous seas swarmed with echini belonging to genera of which no traces have been found in earlier rocks; viz. Holaster, Salenia, Micraster, Salerites, Discoidea, Ananchytes, Cassidulus, &c.
In the Tertiary formations, Spatangus, Scutella, Clypeaster, and other new genera appear, and many of the ancient ones are absent; or at least have not been observed. Of the genera printed above in italics, no living species are known.
ON COLLECTING FOSSIL ECHINODERMATA.
On collecting and developing fossil Echinodermata.—In the previous remarks on the fossil remains of radiated animals, we have pointed out those remains that are the most important and instructive, and should be sought for by the student. Thus, in collecting Crinoids, the receptacle or body should be the principal object of research, and if only detached plates can be extracted from the rock, their relative position should be carefully noted, and the specimens glued to a card or board, in their natural order; and some of the ossicula of the column, and of the arms, or tentacula, be placed with them.