CLAM II 



HOLOTHURIOIDEA 249 



even in the Carboniferous (Eocidaris), but not until the Mesozoic era do the 

 Km.-hiiiuiilr.i completely replace the Palwechinoidea, which become extinct with 

 the Triassic Ti<ir<> 7/ inns. 



All of the Triassic Euechinoidea are true Cidarids, and occur chiefly in the 

 Alps. Some of them are remarkable for their massive spines, which are found 

 in places, as at St. Cassian, very profusely ; perfect tests, however, are rare. 

 The European Lias yields a few Cidarids, and also the oldest known exocyclic 

 Urchins (f/.//ov,/'//.'/"-s /'wider). 



Especially rich in regular Sea-Urchins, as well as in members of the 

 J-:,-liinnnn,lae y 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 

 tfj><if,in,tiil,n>. 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 Sea-Urchins are distributed over the whole world, 

 and <ire particularly plentiful in the Nummulitic Limestone of Europe, 

 Northern Africa, Asia Minor, and India. 



As to phylogenetic relationships, the Perischoechinoida may very properly 

 be regarded as the forerunners and ancestors of the regular Euechinoidea. 

 Among the latter the Cidaridae form the most primitive and stable type ; 

 the Diadematoida, on the other hand, exhibit a much greater aptitude for 

 variation. 



Whether the irregular Euechinoidea are to be regarded as derived from the 

 Silurian Echinocystites, or from the regular Euechinoidea, is an open question. 

 It is obvious, however, that the gnathostomous Echinoconidae are in a number 

 of respects very closely related to the Regulares. The atrophy of teeth in the 

 Echinoneidae certainly gave rise to the Cassidulidae, and subsequent modi- 

 fications led to the derivation of the Ananchytidae and Spatangidae. The 

 Clypeastroids may also be looked upon with much plausibility as a peculiarly 

 modified offshoot of the Echinoconinae, and also as having affinities with the 

 Holectypoid genus Discoidea. The parallelism between ontogeny and phylo- 

 geny as manifested in various groups of the Echinoidea is very striking. 



Class 2. HOLOTHURIOIDEA. Forbes. 



The Holothurians, or Sea-Cucumbers, differ markedly from all other 

 Echinoderms, being characterised by the possession of an elongated, vermi- 

 form body, a terminal anus, and contractile tentacles surrounding the mouth, 

 and by the absence of a test proper. 



Their palaeontological importance is small, since the only structures they 

 possess which are at all capable of preservation are the small integumentary 

 calcifications, occurring in the form of wheels, spicules, spherules, anchors, 

 etc. Such detached calcareous structures are known from the British Carbon- 

 iferous rocks, from the Lias and Dogger of Lorraine, the Upper Jura of 

 Franconia, and the Cretaceous of Bohemia ; they are unquestionably of Holo- 



