TRANSFORMATIONS OF THE ECHINI. 77 



seated. A highly remarkable and important interme- 

 diate form is also known, found in the upper Silurian 

 strata of Dudley (Eucladia Johnsoni), the more impor- 

 tant as but few transitional forms between one order 

 and another have been hitherto discovered. The rela- 

 tion 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 carbon- 

 iferous formation, the no longer sessile Cystoids and 

 Blastoids, with which are associated the Tessellae, more 

 resembling 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 Echinidae are slowly 

 accomplished, and may be traced, step by step, from the 

 Lias, the earliest oolitic formation, to the coral limestone. 

 At first the Cidaridae predominate ; they are joined in the 

 Oolite by the Echinoconidas and Cassidulidae. A sharper 

 distinction 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 Regularae and Endocyclicae, which pri- 

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

 Cidaridae ; while the most perfect Spatangae, 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 



