thansformations of the echin^e. 



77 



have been hitherto discovered, The relation of the star- 

 fish to the sea-urchins is still indistinct. On the other 

 hand, the bridge from the stone-lilies to the sea-urchins 

 is tolerably apparent. The true Crinoids are sessile, and 

 with them are connected, in the carboniferous forma- 

 tion, the no longer sessile Cystoids and Blastoids, with 

 which are associated the Tessellse, more resembhng 

 the sea-urchins. Now the Dyas and Trias are still poor 

 in true Echini; the Oolite, on the contrary, very rich; 

 and in this great period the extraordinarily heterogeneous 

 transformations of the Echinidse are slowly accomplished, 

 and may be traced, step by step, from the Lias, the earliest 

 oolitic formation, to the coral Umestone. At first the 

 Cidaridse predominate; they are joined in the Oolite by 

 the Echinoconidse and Cassidulidae. A sharper distinc- 

 tion of species is characteristic of the later strata of the 

 Upper Jura. 



Desor shows how this development, accompanied by 

 temporary quiescence, is connected with the nature of the 

 sea-bottom at the time. " The law of progress," he says, 

 " is displayed in the circumstance that it is the lowest of 

 the Echinae, the Regularse and Endocyclicse, which pri- 

 marily appear, first in the form of the Tessellae, then as 

 Cidaridas; while the most perfect Spatangse, with the 

 most distinctly marked bilateral form, make their ap- 

 pearance last of all. Between these extremes we find 

 a host of genera and species distinguished from one 

 another by mere shades, so that of two allied genera it is 

 often difficult, nay, impossible, to state which is the more 

 perfect. Progression is only to be shown collectively; in 

 the concrete case it can rarely be demonstrated." 



The Echinae still predominate in the chalk. Recent 



