750 GEOLOGICAL SUCCESSION OF THE ECHINI. 



takable and extremely suggestive of the closer affinities of the Cassiduloids 

 and Galeritidae. 



The Cretaceous period is characterized by the marked development of the 

 Spatangoids proper. Of those with fascioles Micraster and Hemiaster are the 

 first to appear; they recall most forcibly the embryonic and younger stages 

 of Spatangus proper and of its allies, which now form the principal species 

 with those recent Petalosticha which are genetically traced back to the 

 chalk. The generic types characteristic of the upper Cretaceous are already 

 remarkably closely allied to those now living, and many of them are identi- 

 cal. The Cassidulidae, the Ananchytidae, the Schizasteridae, the Brissidae, 

 the Spatangidae, are represented by genera passing through the Tertiaries. 

 identical with those of the present epoch. The types which appear are all 

 closely related to ihose immediately preceding in the lower Cretaceous for- 

 mations. In the upper Cretaceous the earliest Clypcastroids arc suddenly 

 brought into life. They are, it is true, the genera Echinocyamus and Fibu- 

 laria, the embryonic types of the Clypcastroids. and we might imagine a 

 genetic connection between them and the Discoidea on account of the pres- 

 ence of teeth and the more or less rudimentary radiating partitions, and 

 through the Discoidea to the regular Echini. Hut the late appearance of 

 the Clypeastroids, long after the existence of many of the families of the Cas- 

 siduloids and of the Spatangoids, of which they should, according to the the- 

 ory of derivation, have been the ancestors, is one of the principal features in 

 the geological succession of the Echini, which at present can only be ac- 

 counted for by the sudden appearance of the early Spatangoids. 



As soon as we reach the- Tertiary formation the similarity of the fauna to 

 that now in existence becomes quite apparent, and it is more and more dis- 

 similar from the preceding Cretaceous fauna- the nearer we approach the 

 Quaternary period. In the Eocene we find many Spatangoids, — Schizaster. 

 Brissopsis, Eehinocardiuni. Breynia. Brissus. Eupatagus, Spatangus. Maretia, 

 and so forth, which are still well represented in our seas, and suddenly come 

 to light associated with such genera as Conoclypus, Hemiaster, Periaster. 

 Pygorhynchus, and Echinolampas, which have continued to exist from the 

 upper Cretaceous period, and most of which continue also to the present 

 time ; some of them were more numerous in species in earlier times, such 

 as Echinolampas. Pygorhynchus, while Conoclypus does not extend beyond 

 the Eocene. The characteristic genera of the Cretaceous period, the Galeri- 

 tidae and the Dysasteridae, as well as the Ananchytidae, have disappeared, 



