Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/317

 Rh up of many similar parts disposed like Rays around a common centre.

Mr. Agassiz has recently shown, (London and Edin. Phil. Mag. Nov. 1834, p. 369,) that they do not partake of this character, from which the division of radiated animals is named; but that their rays are dissimilar, and not always connected with a uniform centre; and that a bilateral symmetry, analogous to that of the more perfect classes of animals, exists throughout the families of Echini, Asteriæ, and Crinoidea.

The History of the fossil species of Echinidans and Stelleridans has been most beautifully illustrated, in the plates of the Petrefacten of Prof. Goldfuss. Though derived from Strata of various degrees of high antiquity, they are for the most part referred by him to existing Genera.

The family of Echinidans appears to have extended through all Formations, from the Epoch of the Transition series to the present time.

No fossil Stelleridans have yet been noticed in strata more ancient than the Muschelkalk.

As the structure of the fossil species of both these families is so nearly identical with that of existing Echini, and Starfishes, I shall confine my observations respecting fossil animals in the class of Echinoderms to, a family which is of rare occurrence, excepting in a fossil state, and which seems to have prevailed most abundantly in the most ancient fossiliferous formations.