GEOLOGICAL SUCCESSION OF THE ECHINI. 



The earliest Echini known, the Palaechinidae, appear in the Silurian period. 

 They are thus far the only Echini discovered up to the time of the Trias. 

 The number both of the genera and species is comparatively small. In 

 the same formations, associated with them, are found Starfishes, Crinoids, and 

 Ophiurans; as far as described, the differences between these orders, in the 

 earliest formations of the Palaeozoic period, were fully as great as those 

 existing at the present day. All we know regarding the early Echinoderms 

 is. that the Crinoids were infinitely more varied than they are in the subse- 

 quenl formations, and by far the most numerous of the Echinoderms, while 

 the Starfishes, Echini, and Ophiurans were less numerous. This same nu- 

 merical proportion is maintained throughout the Palaeozoic period till we 



come to the Trias, when we are suddenh introduced to a genus which has 

 undergone bill little change to the present day. — the genus Cidaris, — and 

 which takes its greatest development in the Jurassic period. Associated 

 with it, and preceding it in time, we find plates of Echini (Echinothuriae), 

 quite closely allied to the Perischoechinidae (they appear in the Carbonifer- 

 ous period), of which traces have been discovered in the Cretaceous, and of 

 which representatives are still extant in our times. 



What is specially characteristic of the Secondary period — the Trias. Lias, 

 and Jura — is the sudden appearance of types totally dissimilar in structure 

 from any of the Echini found in the geological epoch preceding it. — the 

 sudden appearance of the early type- of Spatangidae, such as the Dysas- 

 teridae, which have absolutely no relationship with the genera preceding 

 them in time; what is particularly remarkable is their appearance before the 

 so-called transition forms between the regular Echini and the Spatangoids. 

 the Galerites. In fact, though the theoretical genesis of the Spatangoids from 

 the tessellate Echini can be most satisfactorily evolved, the data unfortunately 

 do not in the least accord with it. ami the actual occurrence of Collyrites 

 before Holectypus and before the Cassidulidae leaves us completely at a 



