CLASS II HOLOTHURIOIDEA 249 
even in the Carboniferous (Hocidaris), but not until the Mesozoic era do the 
Euechinoidea completely replace the Paleechinoidea, which become extinct with 
the Triassic T7arechinus. 
All of the Triassic Huechinoidea 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 (Galeropygus, Pygaster). 
Especially rich in regular Sea-Urchins, as well as in members of the 
LEchinoneidae, 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 Sea-Urchins are distributed over the whole world, 
and are particularly plentiful in the Nummulitic Limestone of Europe, 
Northern Africa, Asia Minor, and India. 
As to phylogenetic relationships, the Perischoéchinoida may very properly 
be regarded as the forerunners and ancestors of the regular Huechinoidea. 
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 Huechinoidea are to be regarded as derived from the 
Silurian Lchinocystites, or from the regular Huechinoidea, is an open question. 
It is obvious, however, that the gnathostomous Lchinoconidae 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 Hchinoconinae, and also as having affinities with the 
Holectypoid genus Discoidea. 'The parallelism between ontogeny and phylo- 
geny as manifested in various groups of the Hchinoidea 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, 
ete. 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- 
