ENCHINIDANS AND STELLERIDANS. 313 



up of many similar parts disposed like Rays around a com- 

 mon centre. 



Mr. Agassiz has recently shown, (London and Edin. 

 Phil. Mag. Nov. 1834, p. 309,) 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 sym- 

 metry, analogous to that of the more perfect classes of ani- 

 mals, exists throughout the families of Echini, Asteriaa^, and 

 Crinoidea. 



ECHINIDANS AND STELLERIDANS. 



The History of the fossil species of Echinidans and Stel- 

 leridans 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 Star- 

 fishes, I shall confine my observations respecting fossil ani- 

 mals 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 fos- 

 siliferous formations. 



• I found many years ago fossil Echinidans in the Carboniferous lime- 

 stone of Ireland, near Donegal, they are however rare in the Transition for- 

 mation, become more frequent in the Muschelkalk and Lias, and abound 

 throughout the Oolitic and Cretaceous formations. 



VOL. I. — 27 



