CLÄSS I ECHINOIDEA 309 



In the Carboniferous very few Echini are known, and these belong to the 

 Perischoechinoida. The same order is represented by a few types in the 

 Permian which, with a single species of Miocidaris representing the Cidaroida, 

 are the only Echini known. 



In the Trias, Cidarids occur and also the earliest representatives of the 

 Centrechinoida. In the same horizon also occurs Tiarechinus, representing 

 the peculiar order Plesiocidaroida. 



Especially rieh in regulär Sea-urchins, as well as in members of the 

 Echinoneidae, Cassidulidae and Collyritidae are the Middle and Upper Jura 

 of England, France, Germany, Switzerland, the Alps and Northern Africa. 

 The Lower Cretaceous of the same region exhibits no essential change in 

 the Echinoid fauna ; but the advent of large numbers of the Ananchytidae 

 and Spatangidae in the Middle and Upper Cretaceous of Europe, Northern 

 Africa, Asia and North America imparts to these horizons a characteristic 

 appearance. 



During the Tertiary the Cidaridae notably decline, the Echinoconinae 

 become entirely extinct, and the Clypeastroids and Spatangoids advance con- 

 spicuously into the foreground, taking on more and more the semblance of 

 Recent species. Tertiary Echinoids are of world-wide distribution and are 

 particularly plentiful in the Nummulitic Limestone of Europe, Northern 

 Africa, Asia Minor and India. 



As to phylogenetic relationships, it is believed that strueture and develop- 

 ment should be the basis for such studies. While it is earnestly desired that 

 we should find fossils in the proper geological horizons representing every 

 Step in a genealogical sequence, it must be remembered that in the older 

 Paleozoic formations (Silurian and Devonian) Echini are extremely rare. 

 Eecent studies have yielded many new Paleozoic forms and have considerably 

 extended the geological ränge of genera, families and Orders, so that it is not 

 too much to expect that future discoveries will yield material of first 

 importance to a knowledge of the group. Echini are an essentially circum- 

 scribed group and no known type presents a close approach to any other class 

 of Echinoderms. Though the ancestor of the class is unknown, it seems that 

 it might fairly be sought among the Cystids. 



The most primitive known Echinoid structurally is the Ordovician 

 Bothriocidaris, sole representative of its order, which in the adult has 

 characters that appear as stages in development in all other Orders of Echini. 

 Bothriocidaris, with ten columns of ambulacral and five columns of inter- 

 ambulacral plates, in these characters represents the simplest known type. 



The next step structurally is ten columns of ambulacral and ten of inter- 

 ambulacral plates. This strueture is the character of the Cidaroida, 

 Centrechinoida and Exocycloida. Of these Orders the Cidaroida with simple 

 ambulacral plates is certainly the most primitive as well as geologically the 

 oldest. The Centrechinoida typically have Compound plates formed by the 

 coalescence of simple plates. Of this order the Aulodonta are the most 

 primitive group, make the nearest approach to the Cidaroida structurally, and 

 also geologically are the oldest of the order. The Stirodonta as regards the 

 strueture of the lantern (keeled teeth) are further removed from the primitive 

 than the Aulodonta. The Camarodonta are the last expression of differentia- 

 tion of the Centrechinoida in regard to the strueture of the lantern (keeled 

 teeth with wide epiphyses joining in suture over the foramen) and also in 



