GEOLOGICAL SUCCESSION OF THE ECHINI. 749 



loss to account for the appearance of the former genus by derivation. Dur- 

 ing the Trias a few genera, such as Henhcidaris and Hypodiadema, appear-, 

 which are closely allied to the Cidaridae proper ; as early as the lower 

 Jurassic beds we find other generic types, still extant at the present time 

 (Hemipedma), with genera closely allied to the Diadematidae, Echinidae 

 proper, as well as the Cidaridae. These genera (the Pseudodiadematidae) 

 have culminated in the Cretaceous period, and have gradually disappeared 

 during the Tertiaries. The remarkable development of the Cassidulidae is a 

 striking feature of the lower Jurassic beds, especially the occurrence of such 

 genera as Clypeus, Echinobrissus, Pygurus, side by side with the Dysas- 

 teridae of the preceding periods, which disappear in the upper cretaceous 

 beds, though Spatangoids closely allied to them are still found in the Ter- 

 tiaries (Infulaster) ; and even at the present day (Pourtalesia). The greatest 

 development of the Cidaridae proper takes place in the upper Jurassic beds ; 

 they continue through the Cretaceous and Tertiary periods to the present 

 time, and they are still quite numerous in our tropical seas. In the upper 

 Jurassic beds they are, however, associated with a large number of species 

 of Galeritidae, especially Holectypus and Pygaster, and with a number of 

 Echinobrissidae and Dysasteridae. The number of genera common to the 

 Jurassic and the lower beds of the Cretaceous periods is great ; here again 

 we find, as in the passage from the Primary to the .Secondary epochs, 

 genera suddenly appearing (Toxaster and Holaster), closely related to 

 the Dysasteridae, and belonging to a family (the Ananchytidae) which 

 is a very characteristic one of the Cretaceous period, and has survived to the 

 present time, — Paleopneustes, Homolampas. The Galeritidae continue 

 numerous, and are represented in the chalk, not by Galeritidae most closely 

 allied to the Clypeastroids or Spatangoids, but by Galeritidae most closely 

 allied to the Echinidae proper. The early appearance of the Galeritidae, 

 which I have shown to be eminently embryonic Cassidulidae, before the Cas- 

 sidulidae, is one of the anomalies of the genetic succession, at variance with 

 the theoretical derivation. From what we now know of the Galeritidae, 

 since they have been shown to possess teeth and actinal cuts, they evidently 

 are not so far removed from the Desmosticha as we might suppose. Their 

 appearance in the Jurassic period coincides remarkably well with the idea 

 that they form the connecting link, in the sense of sudden transitions, be- 

 tween the Desmosticha and some of the Petalosticha ; for the embryology of 

 Echinolampas has plainly shown us an early Discoidea stage, which is unmis- 



